fix workflow to work with complicated content
Summary:
we were aggressively checking to make sure the event target was a workflow element. instead, just update the event target to the workflow element if necessary.
could probably just do this unconditionally as well.
Test Plan: D3705 works with this in place!
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D3707