Implement order and chaos modes for Remarkup editing

Summary:
Ref T2599.

Implements an "order" mode which fullscreens the editor and reduces distractions, similar to Asana's "focus" mode and GitHub's "zen" mode. This can help users who need fewer distractions get work done.

Implements a "chaos" mode which does the opposite. This can help users who need more distractions to get work done.

Test Plan: Clicked "order" and "chaos" buttons.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2599

Differential Revision: https://secure.phabricator.com/D5735