Fix table backup + delete button on settings/roles/edit/, developer/logs:delete_all etc. by restoring post_key_exists()

For the cause of the issue, see https://github.com/ci-bonfire/Bonfire/commit/be42cff5dc0c490196429fa1b318993dd2c740a1#commitcomment-2349113.

Don't get me wrong :).  I wasn't offended by the removal, and I'm happy to see alternative suggestions.   post_key_exists() seemed to be the least bad solution.  I've revised the API comment to explain the reasoning: 37f1b4f2687d72e960a3c7ad5df8849cdd236ea5.

(Again, I'm not as set on this as the tone of the API comment might suggest - obviously the comment is written with the assumption that the commit is acceptable and gets merged).