[Resolve this issue](https://github.com/coder/internal/issues/506)
Add a mark-all-as-read endpoint which is marking as read all
notifications that are not read for the authenticated user.
Also adds the DB logic.
This PR is part of the inbox notifications topic, and rely on previous
PRs merged - it adds :
- Endpoints to :
- WS : watch new inbox notifications
- REST : list inbox notifications
- REST : update the read status of a notification
Also, this PR acts as a follow-up PR from previous work and :
- fix DB query issues
- fix DBMem logic to match DB
