Event date window

@invisiblefunnel I wanted to send this your way for feedback before I keep going in this direction.

* Overall it creates an endpoint for event digests based on a subscription id.
* I'll add inline notes in context for us to discuss. 
I imagine `/subscription/:id` is a preferable route but didn't immediately see how to add an html endpoint inside of the Grape `Subscriptions` class. Easy? Hard? I bet you have knowledge eggs at the ready.