Autosave in level editor

[faucomte97]

Task Description

As a logged in user I would like it if every time I edited a level, it would autosave so that I don't lose my work.

Acceptance Criteria

• [ ] Game should autosave level editor state on any state change on the DB (only for logged in users)
• [ ] Add a last_saved_at field on the Level model and auto generate names with "Untitled X" convention where X is a number.
• [ ] Make name & owner unique together.

Analytics Requirements

• [ ] None

Copywrite Requirements

• [ ] @lauracumming