Send Devise mailers asynchronously

see: https://github.com/mhfs/devise-async

Configured devise-async sidekiq worker to
  * retry a particular email maximum 3 times
  * notify if 3 retries were exhausted
Make exception_notification work with Sidekiq

Sidkiq handles exceptions raised in workers and notifies
via exception_notification, for which it depends on this
version of exception_notification.

also, correcting the exception_notification config to
send the correct application name in email subject.
Removing ambiguous part of status text