Email digests

* The email template is a bare bones table for now. Perhaps we can put together a fancy email-friendly html template and text fallback.
* I'll add inline comments below
@invisiblefunnel I took a whack at using [Sidekiq::Util.hostname](http://www.rubydoc.info/github/mperham/sidekiq/Sidekiq/Util:hostname) a couple of times with no luck. Possible pebkac