Add Notifications

CROSS-REPO DEPENDENCIES:
- https://github.com/reviewed/the-guide/pull/90

This adds a top level "Notifications" model to our API for things like site-wide banners and notifications. These objects have a lifespan denoted by their start and end dates. They can either have text, in which case they'll appear like the standard bootstrap info header, or an image, which will be the original size.