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 a property of a component on the page that is already published does not change "has_changes"-- Ben has a PR with this fix.
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
Ideally these views would not need a container page to test themselves. It seems to me that they should be just reacting to changes in the model and updating themselves. Part of the problem as we discussed is that they are updating HTML elements that belong to the container page, rather than owning their own rendering. I think it would help the modularity here if they weren't so strictly tied together.