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/lib/feed_manager.rb

@@ -0,0 +1,11 @@
+class FeedManager
+  MAX_ITEMS = 800
+
+  def self.key(type, id)
+    "feed:#{type}:#{id}"
+  end
+
+  def self.filter_status?(status, follower)
+    (status.reply? && !(follower.id = replied_to_user.id || follower.following?(replied_to_user)))
+  end
+end