Make control + enter submit forms if a textarea is focused

Summary:
This is straightforward, except that `form.submit()` does not call onsubmit handlers. Create a `didSyntheticSubmit` event and have everything which listens for form submits listen for it too.

Fixes T704.

Test Plan: Hit control + enter in inline comments, main commetns, Pholio, conpherence. Verified it triggered appropriate JS (workflow / special behaviors).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T704

Differential Revision: https://secure.phabricator.com/D4704