Add basic support for editing project policies

Summary:
This case is unusually complicated because there are more rules than most objects will have.

• Edits are either "joins", "leaves" or "other edits".
• "Joins" require "can join" or "can edit".
• "Leaves" don't require any policy.
• "Other edits" require "can edit".
• You can't edit away your ability to edit.
• You can leave a project that you wouldn't be able to rejoin.

Things I'm going to add:

• Global log of policy changes.
• bin/policy script for undoing policy changes.
• Test coverage for these rules.

Test Plan: Made various project visibility edits with various users, joined / left projects, etc. I'll add more complete coverage in the next diff.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3270