WIP: New Publishing controls on Unit page.

STUD-1707

Known issues:

1. "has_changes" is a bit buggy at the moment. The best way to see "has_changes" change state is to add or delete components on the page. Editing the display name of the unit does not consistently change "has_changes", and editing a property of a component on the page certainly does not. Ben is working on these fixes.
2. The disabled colors for "View Published Version" and "Preview Changes" are not consistent.
3. "Preview" functionality does not work on sandbox-- Feanil to work on next week
4. "Discard changes" is not complete-- will be finished as a separate story.

Notes/Questions:

Sandbox with test course--  http://studio.cahrens.m.sandbox.edx.org/course/slashes:foo+foo+foo
Note to reviewers:
Discard draft to be fully finished in future story.
Display of last edited state is also in a future story. But this controller will be responsible for the display and updates.