R1:b1243e549c62

Allow unsubscription from projects

Summary:
Fixes T4379. Several changes:

- Migrate all project members into subscribers.
- When members are added or removed, subscribe or unsubscribe them.
- Show sub/unsub in the UI.
- Determine mailable membership of projects by querying subscribers.

Test Plan:
- As `duck`, joined a project.
- Added the project as a reviewer to a revision.
- Commented on the revision.
- Observed `duck` receive mail.
- Unsubscribed as `duck`.
- Observed no mail.