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

db/migrate/20180304013859_add_featured_collection_url_to_accounts.rb

@@ -0,0 +1,5 @@
+class AddFeaturedCollectionUrlToAccounts < ActiveRecord::Migration[5.1]
+  def change
+    add_column :accounts, :featured_collection_url, :string
+  end
+end