Avoid more models in old migrations

Although these migrations run fine today, they could break in the future
if changes are made to the current model code.

db/migrate/046_alter_user_roles_and_blocks.rb

@@ -1,6 +1,9 @@
 require "migrate"
 
 class AlterUserRolesAndBlocks < ActiveRecord::Migration[4.2]
+  class UserRole < ActiveRecord::Base
+  end
+
   def self.up
     # the initial granter IDs can be "self" - there are none of these
     # in the current live DB, but there may be some in people's own local