Federate pinned statuses over ActivityPub (#6610)

* Federate pinned statuses over ActivityPub

* Display pinned toots in web UI

Fix #6117

* Fix migration

* Fix tests

* Update outbox_serializer.rb

* Update remove_serializer.rb

* Update add_serializer.rb

* Update fetch_featured_collection_service.rb

spec/lib/activitypub/activity/update_spec.rb

@@ -7,6 +7,7 @@ RSpec.describe ActivityPub::Activity::Update do
     stub_request(:get, actor_json[:outbox]).to_return(status: 404)
     stub_request(:get, actor_json[:followers]).to_return(status: 404)
     stub_request(:get, actor_json[:following]).to_return(status: 404)
+    stub_request(:get, actor_json[:featured]).to_return(status: 404)
 
     sender.update!(uri: ActivityPub::TagManager.instance.uri_for(sender))
   end