R1:01afb50406f2

Allow project editors to remove watchers

Summary: Ref T10054. There is no technical or product reason not to support this, and it is largely analogous to removing subscribers.

Test Plan:
- Removed watchers.
- Removed members.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10054

Differential Revision: https://secure.phabricator.com/D15062