Projects - add "lock membership", which prevents people from leaving

Summary:
Fixes T5603. Puts the toggling of locking membership into the editor so we get exceptions and all that.

I think the dialogue when you try to leave a project that is locked could be a little better maybe? Right now it just says "You can't leave" and "The membership is locked" more or less; should I surface a link to the policy stuff there too?

Test Plan:

• made a project, toggled the "lock" setting, observed stickiness and good transactions being made
• locked a project and tried to leave as a non-editor - got a dialogue letting me know i couldn't
• locked a project and tried to leave as an editor - left successfully

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5603

Differential Revision: https://secure.phabricator.com/D10508