[JRO] Allow queue backend to be configured
[JRO] Change at_notifier to proper command object

`at_notifier -> notify_mentioned_users`

“At notifier” doesn’t convey what the class does and a command object called “notify mentioned users” does (at least a little better).

* Delete `lib/thredded/email_processor.rb`. No longer used.
* Reset the queue_backend after tests run