MUMNOTPLT-26 : Add in schema and repository for Notification.

* Created schema for basic project
* Created a Repository to save and fetch notifications
* Moved Controller to its own package
* Moved application to a correct package
I think this is fine as is, especially at this early point in the project.

Flagging, however, for thought about whether we need to be using a synthetic identifier for users rather than the human-facing username.  Realities around usernames change but the underlying identity remains the same?