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