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, chad

Reviewed By: chad

Maniphest Tasks: T9019

Differential Revision: https://secure.phabricator.com/D14869