Merge branch 'follow' into 'master'

Follow

See merge request !425

app/serializers/follow_serializer.rb

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