background email processing with sidekiq

Sidekiq require redis.  This is a pretty good How To from Digital Ocean https://www.digitalocean.com/community/articles/how-to-install-and-use-redis

I created the Blast model so that admins could keep track of the progress of emails as they're being processed in the background. I expect this model to evolve over time.

The EmailWorker has retries set to false so that targets don't get multiple emails do to a failure somewhere else. Logging can probably be improved in the future.

I also added the ability to launch and test campaigns from the show page.

Sidekiq monitoring is enabled for admins at /sidekiq

Some of the relevant pages:

https://github.com/mperham/sidekiq/wiki/Monitoring
https://github.com/mperham/sidekiq/wiki/Error-Handling

And a link to the wiki home for sidekiq
https://github.com/mperham/sidekiq/wiki

In response to enhancement issue #51