Prevent double-submit with control + enter

Summary: Fixes T2833. Lock forms and prevent double submit on control + enter.

Test Plan: Mashed control + enter a whole bunch in Maniphest, got one comment instead of several.

Reviewers: btrahan, edward

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2833

Differential Revision: https://secure.phabricator.com/D5473