Transactions - make ctrl + enter submit ajax-style

Summary: We were correctly invoked a didSyntheticSubmit event on the form, but nothing was listening to it. Re-jigger the workflow submit code a tad so an onsyntheticsubmit event handler can be written to fill this gap. Fixes T4669.

Test Plan: edited comments, submitting both via clicking the button and apple + enter LIKE A BOSS

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4669

Differential Revision: https://secure.phabricator.com/D8961