Add Firebase user support

[CSchmitz81]

Add user tracking to Firebase data, so additions/changes/feedback is tied to a specific user. Users should be able to register a new user (username, password, & avatar), login, logout, and update avatar (manually entering a url or selecting one of ours from a list is fine). This may be made simpler by using FirebaseUI Auth. We don't need to implement this functionality for when Firebase is disabled, although we should have a reasonable fallback (ex. hide anything related to login).