Add scheduled sender task detail

![all the small things](http://stream1.gifsoup.com/view/109816/tom-all-the-small-things-o.gif)

All the small things, kind of. I'm just slightly worried that if for some reason these tasks are run in parallel (or there's a deploy screw up and we have two celery's running with `--beat`) then no only will this result in multiple messages being sent but also potentially screwing with the underlying database and forward people to a sequence number they're not supposed to be at yet.

Mostly raising these concerns because these things happen regularly and this is why a scheduler is kind of hard, it's the locking bits.