* Whenever a remote keypair changes, unfollow them and re-subscribe to them

In Mastodon (it could be different for other OStatus or AP-enabled software),
a keypair change is indicative of whole user (or instance) data loss. In this
situation, the “new” user might be different, and almost certainly has an empty
followers list. In this case, Mastodon instances will disagree on follower
lists, leading to unreliable delivery and “shadow followers”, that is users
believed by a remote instance to be followers, without the affected user
knowing.

Drawbacks of this change are:
1. If an user legitimately changes public key for some reason without losing
   data (not possible in Mastodon at the moment), they will have their remote
   followers unsubscribed/re-subscribed needlessly.
2. Depending of the number of remote followers, this may generate quite some
   traffic.
3. If the user change is an attempt at usurpation, the remote followers will
   unknowingly follow the usurper. Note that this is *not* a change of
   behavior, Mastodon already behaves like that, although delivery might be
   unreliable, and the usurper would not have known the former user's
   followers.

* Rename ResubscribeWorker to RefollowWorker

* Process followers in batches

* Add ActivityPub inbox

* Handle ActivityPub deletes

* Handle ActivityPub creates

* Handle ActivityPub announces

* Stubs for handling all activities that need to be handled

* Add ActivityPub actor resolving

* Handle conversation URI passing in ActivityPub

* Handle content language in ActivityPub

* Send accept header when fetching actor, handle JSON parse errors

* Test for ActivityPub::FetchRemoteAccountService

* Handle public key and icon/image when embedded/as array/as resolvable URI

* Implement ActivityPub::FetchRemoteStatusService

* Add stubs for more interactions

* Undo activities implemented

* Handle out of order activities

* Hook up ActivityPub to ResolveRemoteAccountService, handle
Update Account activities

* Add fragment IDs to all transient activity serializers

* Add tests and fixes

* Add stubs for missing tests

* Add more tests

* Add more tests

* Do not raise unretryable exceptions in ResolveRemoteAccountService

* Removed fatal exceptions from ResolveRemoteAccountService

Exceptions that cannot be retried should not be raised. New exception
class for those that can be retried (Mastodon::UnexpectedResponseError)

* Refactor ResolveRemoteAccountService

* Remove trailing whitespace

* Use redis locks around critical ResolveRemoteAccountService code

* Add test for race condition of lock

* Add Request class with HTTP signature generator

Spec: https://tools.ietf.org/html/draft-cavage-http-signatures-06

* Add HTTP signature verification concern

* Add test for SignatureVerification concern

* Add basic test for Request class

* Make PuSH subscribe/unsubscribe requests use new Request class

Accidentally fix lease_seconds not being set and sent properly, and
change the new minimum subscription duration to 1 day

* Make all PuSH workers use new Request class

* Make Salmon sender use new Request class

* Make FetchLinkService use new Request class

* Make FetchAtomService use the new Request class

* Make Remotable use the new Request class

* Make ResolveRemoteAccountService use the new Request class

* Add more tests

* Allow +-30 seconds window for signed request to remain valid

* Disable time window validation for signed requests, restore 7 days
as PuSH subscription duration (which was previous default due to a bug)

Rename Activitypub to ActivityPub

* OEmbed support for PreviewCard

* Improve ProviderDiscovery code failure treatment

* Do not crawl links if there is a content warning, since those
don't display a link card anyway

* Reset db schema

* Fresh migrate

* Fix rubocop style issues
Fix #1681 - return existing access token when applicable instead of creating new

* Fix test

* Extract http client to helper

* Improve oembed controller

* Fix #2119 - Whenever about to send a HTTP request, normalize the URI

* Add test for IDN request in FetchLinkCardService

* Perform IDN normalization on domains before they are stored in the DB

* Fix possibility of unrightful webfinger redirect

* Add more tests for FollowRemoteAccountService

* Refresh local info for remote accounts when webfinger returns new values

It only refreshes account info if one of the URLs or the public-key changes,
in which cases it refreshes the full info, re-downloading the feeds from that
user.

Some special handling should probably be done when the public key changes,
but I have been unable to find any use for it in Mastodon yet.

* Re-fetch remote users we aren't subscribed to.

This might induce performance issues, we might want to only do that for users
we explicitly attempted to subscribe but failed to.

* Refactor changes

* Do not refresh existing remote account details more than once a day

* Avoid re-fetching webfinger info in tests unless otherwise specified

The async action is send before persist, account.id not yet generated

Pull queue receive 'nil' so no profile update.

created via remote look-up when domains differ but point to the same resource

all required resource links (profile page, salmon, atom feed, magic key)

This is a big one, so let me enumerate:

Accounts as well as stream entry pages now contain Link headers that
reference the Atom feed and Webfinger URL for the former and Atom entry
for the latter. So you only need to HEAD those resources to get that
information, no need to download and parse HTML <link>s.

ProcessFeedService will now queue ThreadResolveWorker for each remote
status that it cannot find otherwise. Furthermore, entries are now
processed in reverse order (from bottom to top) in case a newer entry
references a chronologically previous one.

ThreadResolveWorker uses FetchRemoteStatusService to obtain a status
and attach the child status it was queued for to it.

FetchRemoteStatusService looks up the URL, first with a HEAD, tests
if it's an Atom feed, in which case it processes it directly. Next
for Link headers to the Atom feed, in which case that is fetched
and processed. Lastly if it's HTML, it is checked for <link>s to the Atom
feed, and if such is found, that is fetched and processed. The account for
the status is derived from author/name attribute in the XML and the hostname
in the URL (domain). FollowRemoteAccountService and ProcessFeedService
are used.

This means that potentially threads are resolved recursively until a dead-end
is encountered, however it is performed asynchronously over background jobs,
so it should be ok.

respect hub.lease_seconds (fix #46)