Allow administrative editing of project resources

Summary:
Currently, you can only edit your own affiliation to projects. Enable users to
be managed in a more reasonable batched way.

I'll lock this down to admins/owners and add a transaction log at some point.

Test Plan: Edited project affiliations. Verified Herald still works.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 677