Write a "resign" audit relationship even if actor has no relationship

Summary: Ref T4896. I got this logic slightly wrong when porting it over: we always want to write this relationship, to allow members of a project with an audit request against a commit to resign and get it out of their queue.

Test Plan:
  - Resigned from a commit with an existing relationship.
  - Resigned from a commit with no existing relationship, saw one added.

Reviewers: btrahan, joshuaspence, mbishopim3

Reviewed By: mbishopim3

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10137