searchable: don't automatically include id, created_at, updated_at.

Don't make search methods on models call super in order to search
certain default attributes (id, created_at, updated_at). Simplifies some
magic.

app/models/favorite.rb

@@ -12,8 +12,7 @@ class Favorite < ApplicationRecord
   end
 
   def self.search(params)
-    q = super
-    q = q.search_attributes(params, :post)
+    q = search_attributes(params, :id, :post)
 
     if params[:user_id].present?
       q = q.for_user(params[:user_id])