* Custom emoji

- In OStatus: `<link rel="emoji" name="coolcat" href="http://..." />`
- In ActivityPub: `{ type: "Emoji", name: ":coolcat:", href: "http://..." }`
- In REST API: Status object includes `emojis` array (`shortcode`, `url`)
- Domain blocks with reject media stop emojis
- Emoji file up to 50KB
- Web UI handles custom emojis
- Static pages render custom emojis as `<img />` tags

Side effects:

- Undo #4500 optimization, as I needed to modify it to restore
  shortcode handling in emojify()
- Formatter#plaintext should now make sure stripped out line-breaks
  and paragraphs are replaced with newlines

* Fix emoji at the start not being converted

* Fix an error when actor json couldn't be fetched in ResolveRemoteAccountService

* Add specs

* Improve OpenGraph tags for about pages

* Add thumbnail admin setting

* Fix error

* Fix up

* Fix ActivityPub handling of replies when LOCAL_DOMAIN ≠ WEB_DOMAIN (#4895)

For all intents and purposes, `local_url?` is used to check if an URL refers
to the Web UI or the various API endpoints of the local instances. Those things
reside on `WEB_DOMAIN` and not `LOCAL_DOMAIN`.

* Change local_url? spec, as all URLs handled by Mastodon are based on WEB_DOMAIN

- Return all images as og:image
- Return videos as og:image (preview) and og:video
- Return profile:username on profiles

When a new user confirms their e-mail, bootstrap their home timeline
by automatically following a set of accounts. By default, all local
admin accounts (that are unlocked). Can be customized by new admin
setting (comma-separated usernames, local and unlocked only)

In before, the method uses stream_entry id as status id, so replied status was wrongly selected.

This PR uses StatusFinder which was introduced with `Api::Web::EmbedsController`.

* Fix language filter codes

CLD3 returns BCP-47 language identifier, filter settings expect
identifiers in the ISO 639-1 format. Convert between formats,
and exclude duplicate languages from filter choices (zh-CN->zh)

* Fix zh name

* Decouple Status#local? from uri being nil

* Replace on-the-fly URI generation with stored URIs

- Generate URI in after_save hook for local statuses
- Use static value in TagManager when available, fallback to tag format
- Make TagManager use ActivityPub::TagManager to understand new format
- Adjust tests

* Use other heuristic for locality of old statuses, do not perform long query

* Exclude tombstone stream entries from Atom feed

* Prevent nil statuses from landing in Pubsubhubbub::DistributionWorker

* Fix URI not being saved (#4818)

* Add more specs for Status

* Save generated uri immediately

and also fix method order to minimize diff.

* Fix alternate HTML URL in Atom

* Fix tests

* Remove not-null constraint from statuses migration to speed it up

* Show pinned statuses only in the top of the profile page

* Refactor AccountsController#show_pinned_statuses?

* Raise an error for remote url in StatusFinder

Previous implementation had allowed remote url with status id which also exists on local.

Then that bug leads /api/web/embed to return wrong embed url.

* Fix oembed_controller_spec

* Make PreviewCard records reuseable between statuses

**Warning!** Migration truncates preview_cards tablec

* Allow a wider thumbnail for link preview, display it in horizontal layout (#4648)

* Delete preview cards files before truncating

* Rename old table instead of truncating it

* Add mastodon:maintenance:remove_deprecated_preview_cards

* Ignore deprecated_preview_cards in schema definition

* Fix null behaviour

* Don't process ActivityPub payload if signature is invalid

* Fix style issue

* Forward ActivityPub creates that reply to local statuses

* Fix test

* Fix wrong signers

- Use statuses controller for embeds instead of stream entries controller
- Prefer /@:username/:id/embed URL for embeds
- Use /@:username as author_url in OEmbed
- Add follow link to embeds which opens web intent in new window
- Use redis cache in development
- Cache entire embed

Requires moving Atom rendering from DistributionWorker (where
`stream_entry.status` is already nil) to inline (where
`stream_entry.status.destroyed?` is true) and distributing that.

Unfortunately, such XML renderings can no longer be easily chained
together into one payload of n items.

* Add handling of Linked Data Signatures in payloads

* Add a way to sign JSON, fix canonicalization of signature options

* Fix signatureValue encoding, send out signed JSON when distributing

* Add missing security context

* Allow multiple pinned statuses to be shown and make them be ordered by pinned date

* Set timestamps NOT NULL

* Make single-line pinned_statuses

* Spec for pinned_statuses

* Remove redundant empty line

* Pinned statuses

* yarn manage:translations

* Process Create / Announce activity in FetchRemoteStatusService

* Use activity URL in ActivityPub for reblogs

* Redirect to the original status on StatusesController#show