R1:3238f1e09142

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…