Prevent Send on Enter in Fullscreen Remarkup Mode

Summary: Fixes T12138. Test for the presence of being in fullscreen mode, and disable send on enter if present. Side note, I'd love a first class "hasClass" type Javelin function.

Test Plan:

• Go to Conpherence
• Type some smack, see it send on enter
• Go fullscreen like a boss
• Let the words flow
• Close fullscreen, then send on enter.
• (might be nice someday to add a "submit" button to fullscreen editor)

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12138

Differential Revision: https://secure.phabricator.com/D17590