* Add REST API for creating an account

The method is available to apps with a token obtained via the client
credentials grant. It creates a user and account records, as well as
an access token for the app that initiated the request. The user is
unconfirmed, and an e-mail is sent as usual.

The method returns the access token, which the app should save for
later. The REST API is not available to users with unconfirmed
accounts, so the app must be smart to wait for the user to click a
link in their e-mail inbox.

The method is rate-limited by IP to 5 requests per 30 minutes.

* Redirect users back to app from confirmation if they were created with an app

* Add tests

* Return 403 on the method if registrations are not open

* Require agreement param to be true in the API when creating an account

Fixes #9580

* Add moderation warnings

Replace individual routes for disabling, silencing, and suspending
a user, as well as the report update route, with a unified account
action controller that allows you to select an action (none,
disable, silence, suspend) as well as whether it should generate an
e-mail notification with optional custom text. That notification,
with the optional custom text, is saved as a warning.

Additionally, there are warning presets you can configure to save
time when performing the above.

* Use Account#local_username_and_domain

* Add spec for RelayPolicy

* Add specs for SubscriptionPolicy

* Add specs for SettingsPolicy

* Add specs for TagPolicy

* Add specs for ReportPolicy

* Add failing test for windows-1251 link cards

* Ignore low-confidence CharlockHolmes guesses

Fixes #9466

* Fix no method error when charlock holmes cannot detect charset

* Error message for avatar image that's too large. #9204

* Code climate/formatting

* Removed avatar error message

* Moved valid image dimentions check to update service

* removed unnescessary begin block

* code climate formatting

* code climate indent fix

* Make custom emoji domains case sensitive #9351

* Fixup style in downcase_domain to comply with codeclimate.

* switch if! to unless

* Don't use transactions, operate in batches.

Also revert spurious schema change.

* Add specs for ActivityPub::CollectionsController#show

* Raise ActiveRecord::RecordNotFound

Raising ActiveRecord::NotFound raises NameError: uninitialized constant
ActiveRecord::NotFound.

Since dots are not allowed in username,
this spec is no longer needed.