Basic notifications handling.

Here's the new handler + some unit tests.

@nasthagiri @flowerhack @mmprandom 

LMS-11163
Consider using the API on the `CourseRerunUIStateManager` class, or even the `CourseActionUIStateManager` class in your testing here.  They are meant to provide a layer of abstraction so higher layer code (including these tests) do not directly call the Django model APIs.  Theoretically, this would allow us to change the persistence layer without needing to change all the callers.