Try to act with the correct voice in Asana sync

Summary:
Ref T2852.

Before trying related users, try using the feed story's actor. This is the most correct voice to act in.

Test Plan: Ran feed/republish.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6305