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/follow.rb

@@ -1,9 +1,9 @@
 class Follow < ActiveRecord::Base
+  include Streamable
+
   belongs_to :account
   belongs_to :target_account, class_name: 'Account'
 
-  has_one :stream_entry, as: :activity
-
   validates :account, :target_account, presence: true
   validates :account_id, uniqueness: { scope: :target_account_id }
 
@@ -16,22 +16,10 @@ class Follow < ActiveRecord::Base
   end
 
   def object_type
-    target.object_type
-  end
-
-  def content
-    self.destroyed? ? "#{self.account.acct} is no longer following #{self.target_account.acct}" : "#{self.account.acct} started following #{self.target_account.acct}"
+    :person
   end
 
   def title
-    content
-  end
-
-  def mentions
-    []
-  end
-
-  after_create do
-    self.account.stream_entries.create!(activity: self)
+    self.destroyed? ? "#{self.account.acct} is no longer following #{self.target_account.acct}" : "#{self.account.acct} started following #{self.target_account.acct}"
   end
 end