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.
• Resubscribed as duck.
• Mail again.
• Joined/left project, checked sub/unsub status.
• Ran migration, looked at database.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, asherkin

Maniphest Tasks: T4379

Differential Revision: https://secure.phabricator.com/D8189