Remove "Former" project members

Summary:
This is a needlessly confusing/complex feature that I originally wrote sort of
speculativley. I think we can better serve what little need may exist here with
project feeds.

I'm probably going to get rid of or deemphasize "role" too and just add "Join
Project" and "Leave Project" buttons.

Test Plan: Viewed project list, project profile. Edited project profile and
affiliation.

Reviewers: btrahan, jungejason, zeeg

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T681

Differential Revision: 1228