Add basic Watching filter to /projects/
Summary: Ref T12707. Adds a simple filter for the viewer if logged in.
Test Plan: Watch a project, click on watching list, see project I'm watching.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: epriestley
Maniphest Tasks: T12707
Differential Revision: https://secure.phabricator.com/D17873