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