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
I'm a bit surprised by the way you modeled this. I would have expected the publisher panel to be its own view file, and that the logic for handling the preview button would live in the container page itself. In fact, that logic may need to be shared with the outline page once we add bulk publishing there too. Having a controller seems useful but it seems to belong only to the publisher panel.

Having everything in a single 'subviews' class just makes it harder to untangle and reason about (to me, at least). It also seems strangely tied to the container page by being called container_subviews. Finally, it seems odd that subviews would live in the pages folder. My intention with the folder was that it would contain all of the pages. All the non-page views just live under views at the moment. 

Can you explain why you went with this approach.