R1:8463ad265960

Replace subscribe/unsubscribe for projects with explicit mail setting

Summary:
Ref T10054. Ref T6113. Users can currently subscribe to projects, which causes them to receive:

# mail about project membership changes, description changes, etc; and
# mail to the project, e.g. when the project is added as a subscriber on a task, or a reviewer on a revision.

Almost no one cares about (1), and after D15061 you can use Herald to get this stuff if you really want it. (It will get progressively more annoying…