R1:3a3155426898

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…