[JRO] Add `Q` gem. Async @ notifications

TODO: Allow the Q configuration to be dictated by the parent
application.

* Extract marking users as notified to a command object instead of a
  private method
* The at notifications being put behind an async queue will speed up the
  creation of posts. Good times.
* Add a `jobs` folder into `./app/`
[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
[JRO] Rename and move private_topic_notifier

Another command object living in the /lib junk drawer.
[JRO] Notify users of private topics on create

* Use a guard clause instead of wrapping code in a conditional on the
  topics controller
[JRO] Notify private topic users behind queue

* Added a job to pull that work into the worker queue
* Move `at_notification_extractor` into commands dir
* Stick to some conventions - commands respond to `run` instance method
* Remove `require`s of the classes moved from lib to commands - bc they’re now auto-required.
[JRO] Topic mailer no longer used

This had been moved to PrivateTopicMailer
[JRO] More available config options bubbled up

Instead of digging into the libraries in the gem certain configuration
options needed to be bubbled up to the Thredded config layer. These are:

```ruby
Thredded.queue_backend
Thredded.queue_memory_log_level
Thredded.queue_inline
```

The readme was lacking some info so ... that needed fixing
[JRO] Clean up after changing singleton state
[JRO] Update CHANGELOG and README