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

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3182