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

app/lib/activitypub/activity/remove.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class ActivityPub::Activity::Remove < ActivityPub::Activity
+  def perform
+    return unless @json['origin'].present? && value_or_id(@json['origin']) == @account.featured_collection_url
+
+    status = status_from_uri(object_uri)
+
+    return unless status.account_id == @account.id
+
+    pin = StatusPin.find_by(account: @account, status: status)
+    pin&.destroy!
+  end
+end