Simplify ProjectQuery handling of viewer membership

Summary:
Ref T10010. Currently, we do an unusual JOIN to make testing for viewer membership in projects a little cheaper.

This won't work as-is once we have subprojects, so standardize, simplify, and cover it with more tests for now. (I may be able to get a similar optimization later, but want a correct implementation first.)

Test Plan:
This change should create no behavioral differences.

• Added tests.
• Ran tests.
• Viewed projects.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D14859