Allow users to seach for projects by watcher

Summary:
Ref T10349. This capability didn't make a ton of sense when you had to be a member to watch a project and watch rules were simple, but makes more sense now.

A particular use case might be finding all the stuff you're watching so you can prune it.

Test Plan: Searched for stuff I was watching, got accurate results.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10349

Differential Revision: https://secure.phabricator.com/D15289