Send Devise mailers asynchronously

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

Configured devise-async sidekiq worker to
  * retry a particular email maximum 3 times
  * warn 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

Success! and Failure! can be misleading
to the admin, all we can unambiguously
state is that the batch invitation CSV
was processed.
Removing this config, as it's replaced on deploy
Merge pull request #175 from alphagov/send-devise-mailers-async

Send devise mailers async