Transactions - fix edit submit URI

Summary:
tracing this a bit, in core/behavior-form.js we end up doing form.submit() on this form. Before this patch, the form had no submitURI set so it ended up doing a page load. Now, we set the submitURI and (at least) the form is actually submitted and data is saved. Ref T4669.

Not sure how come the syntheticSubmit even isn't getting picked up though?

Test Plan: hit apple + enter to submit comment edit. page reloaded and my edit persisted!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4669

Differential Revision: https://secure.phabricator.com/D8952