Add a synthetic DOM event to JX.Workflow for form submission

Summary: Currently, when a dialog is submitted the Workflow itself emits an event but no DOM event is emitted. The workflow event is fine for handlers which only use JS, but there's currently no way for a handler to act more like a normal form handler. This event gives normal form handlers a way to capture Workflow submits and muck around with form contents, etc.

Test Plan: In a future diff, edited policies via a Workflow dialog.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7295