* ActivityPub migration procedure
Once one account is detected as going from OStatus to ActivityPub,
invalidate WebFinger cache for other accounts from the same domain
* Unsubscribe from PuSH updates once we receive an ActivityPub payload
* Re-subscribe to PuSH unless already unsubscribed, regardless of protocol
* 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
* Improve webfinger templates and make tests more flexible
* Clean up AS2 representation of actor
* Refactor outbox
* Create activities representation
* Add representations of followers/following collections, do not redirect /users/:username route if format is empty
* Remove unused translations
* ActivityPub endpoint for single statuses, add ActivityPub::TagManager for better
URL/URI generation
* Add ActivityPub::TagManager#to
* Represent all attachments as Document instead of Image/Video specifically
(Because for remote ones we may not know for sure)
Add mentions and hashtags representation to AP notes
* Add AP-resolvable hashtag URIs
* Use ActiveModelSerializers for ActivityPub
* Clean up unused translations
* Separate route for object and activity
* Adjust cc/to matrices
* Add to/cc to activities, ensure announce activity embeds target status and
not the wrapper status, add "id" to all collections
