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

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

lib/tasks/subscriptions.rake

@@ -5,7 +5,7 @@ namespace :subscriptions do
     accounts = Account.where('(select count(f.id) from follows as f where f.target_account_id = accounts.id) = 0').where.not(domain: nil)
 
     accounts.each do |a|
-      a.subscription(api_subscription_url(a.id)).unsubscribe
+      a.subscription('').unsubscribe
       a.update!(verify_token: '', secret: '')
     end
   end