Add a class to the Differential banner when unsubmitted/unsaved changes are present

Summary: Ref T12733. This adds classes for unsubmitted/unsaved changes, and lays some groundwork for additional buttons.

Test Plan:

• Added, edited, deleted comments.
• Saw bar background color update appropriately.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12733

Differential Revision: https://secure.phabricator.com/D18045