R1:4a8495495738

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…