Add ActivityPub inbox (#4216)

* 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

config/routes.rb

@@ -51,6 +51,7 @@ Rails.application.routes.draw do
     resource :follow, only: [:create], controller: :account_follow
     resource :unfollow, only: [:create], controller: :account_unfollow
     resource :outbox, only: [:show], module: :activitypub
+    resource :inbox, only: [:create], module: :activitypub
   end
 
   get '/@:username', to: 'accounts#show', as: :short_account