Fix subscriptions:clear task, refactor feeds, refactor streamable activites

and atom feed generation to some extent, as well as the way mentions are
stored

app/models/concerns/targetable.rb

@@ -0,0 +1,9 @@
+module Targetable
+  extend ActiveSupport::Concern
+
+  included do
+    def object_type
+      :object
+    end
+  end
+end