R1:42461b5f06f4

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.