Basic notifications handling.

Here's the new handler + some unit tests.

@nasthagiri @flowerhack @mmprandom 

LMS-11163
Also, in re-reading the discussion and document on the wiki page (https://edx-wiki.atlassian.net/wiki/display/LMS/CourseStatusStore+API+Design), I now remember that we decided to delete the entry from the CourseRerunState store once it was no longer needed by the UI.

So if we honor that decision, instead of making an update call here, we should do a `find_first` followed by a `delete` call.