Fix group boards migration name

This name already existed in EE, so we rename the CE version to make them
distinct.

db/migrate/20180227182112_add_group_id_to_boards.rb → db/migrate/20180227182112_add_group_id_to_boards_ce.rb

-class AddGroupIdToBoards < ActiveRecord::Migration
+class AddGroupIdToBoardsCe < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   disable_ddl_transaction!