Add worker queue: Sidekiq

@amoslanka 

> **BLOCKED until a worker server is set up or we decide to pay for another process on Heroku**

> - Includes a middleware to automatically send any exceptions to Rollbar. 
- Rollbar is also configured to use sidekiq for delivery exceptions to its service using the `exception_reporting` queue.
- Includes a user lifecycle mailer worker that will use the "email" queue.