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/streamable.rb

@@ -0,0 +1,31 @@
+module Streamable
+  extend ActiveSupport::Concern
+
+  included do
+    has_one :stream_entry, as: :activity
+
+    def title
+      super
+    end
+
+    def content
+      title
+    end
+
+    def target
+      super
+    end
+
+    def object_type
+      :activity
+    end
+
+    def thread
+      super
+    end
+
+    after_create do
+      self.account.stream_entries.create!(activity: self)
+    end
+  end
+end