Cleaner fullscreen / preview states for Remarkup bar

Summary: General CSS and usability touchup of the Remarkup bar states for fullscreen and preview. Larger fonts, more spacing, some hint of the underlying page. Disable buttons that can't be used in preview mode.

Test Plan:
Formal test coming with mobile, browsers. This is a kick the tires upload.

{F4283448}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17563