Allow projects to be "watched", sort of a super-subscribe

Summary:
Ref T4967. Adds a "Watch" relationship to projects, which is stronger than member/subscribed.

Specifically, when a task is tagged with a project, we'll include all project watchers in the email/notifications. Normally we don't include projects unless they're explicitly CC'd, or have some other active role in the object (like being a reviewer or auditor).

This allows you to closely follow a project without needing to write a Herald rule for every project you care about.

Test Plan:
  - Watched/unwatched a project.
  - Tested the watch/subscribe/member relationships:
    - Watching implies subscribe.
    - Joining implies subscribe.
    - Leaving implies unsubscribe + unwatch.
    - You can't unsubscribe until you unwatch (slightly better would be unsubscribe implies unwatch, but this is a bit tricky).
  - Watched a project, then recevied email about a tagged task without otherwise being involved.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4967

Differential Revision: https://secure.phabricator.com/D9185