Switch User-Merge to a direct method on the User object

[uberbrady]

(Copypasta'd the following from: #7 upon which this branch is based)

This is 'stacked' against the refactor_logging_squashed_rebased branch - and unifies the way we do User Merges whether they be from the CLI or from the GUI.

Now, $user->merge($bad_user) is now a new method on User. I had to get a little clever with callbacks to be able to report the same information to the CLI tool that we had before, but now there's one base of code to do all user-merging.

There was a subtle difference between how we did the CLI-merges versus the GUI merges - one would do a conventional delete() - causing a Delete action_log entry, and one would do a simple save() with deleted_at set to NOW() - which does not add an action_log. I've chosen to standardize on the "conventional delete" method, because I think the logs are clearer that way. Because of that, I needed to modify one test to reflect new, additional action_log entry.