Apply "accept" and "resign" actions with transactions

Summary: Ref T4896. Applies these actions using new transaction stuff.

Test Plan:
  - Accepted and raised concern with my own commit, verifying the special project/package behavior.
  - Accepted and raised concern with another author's commit, verifying the authority-over-packages/projects behavior.
  - Accepted a commit I was not affiliated wiht, verifying the "join as an auditor" behavior.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10125