R1:fbf6d967fffa

Add getMemberPHIDs() and loadMemberPHIDs() to PhabricatorProject

Summary:
I want to:

- move the membership storage to edges
- remove the concepts of "roles" (which are decorative text only) and "owners" (which will be replaced with policy-based controls)

This moves us a step closer to that by reducing the use of ProjectAffiliation outside of the class.

Test Plan: Loaded project profile. Called `project.query`. Joined and left a project.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran…