R1:1c572d1da51f

Implement a "Project Members" object policy rule

Summary:
Fixes T9019. Pretty much ripped from D14467. I added the "policy hint" stuff so that you can create a project with this policy immediately.

I really dislike how the "hint" code works, but we //almost// never need to use it and the badness feels fairly well-contained.

Also pick up a quick feedback fix from D14863.

Test Plan:
- Added test coverage, got it to pass.
- Created a project with "Visible To: Project Members".

Reviewers: joshuaspence…