Make Cmd+Enter work to submit a form

Summary:
Fixes T3010.

`keydown` is what you're supposed to use for special keys like `Enter`; `keypress` didn't work for me for Cmd+Enter on Chrome.

Test Plan: Submitted a Differential field with Cmd+Enter on Chrome Mac. Also made sure Ctrl+Enter still works and Opt+Enter does not.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T3010

Differential Revision: https://secure.phabricator.com/D8699