Add new container page that can display nested xblocks

This is the changes for STUD-1244, which introduces the ability for Studio to display
arbitrarily nested xblocks. In this change, a new container page is introduced which can
display nested xblocks. In particular, the xblock type of 'vertical' is special cased
to be shown inline as a collapsible section. The unit page is mostly unchanged, except
that container xblock's are shown as a link to their container page, rather than being
shown inline.
Update the container reference
updated unit page styling for xblock-type-containers, enabling container blocks on a unit page to link to their corresponding container page
First cut at Jasmine unit tests
Fix bugs caused by rebase.
Fix unit test failures.
Add tests for the new container xblock API
Fix dependencies on the XBlock runtime.
Apply changes based on Christina's review
Update the documentation and add a change log entry.