Merge branch 'user_followers' into 'master'

add follower user to follow serializer

See merge request !374

app/serializers/follow_serializer.rb

 class FollowSerializer < ActiveModel::Serializer
   cache key: 'follow', expires_in: 24.hours
-  attributes :id, :user_id, :followable
+
+  def follower
+    object.user
+  end
+
+  attributes :id, :follower, :followable
 end