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
