R1:cb0bb8165d83

Add a Join / Leave button to Projects

Summary: Make it easy to join or leave (well, slightly less easy) a project.
Publish join/leave to feed. Fix a couple of membership editor bugs.

Test Plan: Joined, left a project.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T681

Differential Revision: https://secure.phabricator.com/D1485