Issue #262 Drupal 7.28 release backports

PR for https://github.com/backdrop/backdrop-issues/issues/262

Replacement PR for https://github.com/backdrop/backdrop/pull/342
Looks like this need to be response['old'] and response['new'], otherwise JavaScript is going to think you're talking about a subitem keyed on a variable `old`, which doesn't exist and it'll throw a JS error. Looks like the original patch included quotes, this was probably introduced in the manual porting:

https://www.drupal.org/files/issues/use-of-reserved-word-2245331-1.patch