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

app/serializers/activitypub/update_serializer.rb

@@ -1,10 +1,14 @@
 # frozen_string_literal: true
 
 class ActivityPub::UpdateSerializer < ActiveModel::Serializer
-  attributes :type, :actor
+  attributes :id, :type, :actor
 
   has_one :object, serializer: ActivityPub::ActorSerializer
 
+  def id
+    [ActivityPub::TagManager.instance.uri_for(object), '#updates/', object.updated_at.to_i].join
+  end
+
   def type
     'Update'
   end