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.
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.
Apply next round of code review changes
Fix typo
Update to Marco's latest xblock class names.
Yet more fixes
Clean up naming
visual style adjustment for collapsed nesting level xblocks on the container page
added window styling class and h2 styling to ensure visual consistency for components viewed on the container page.
Fix expand/collapse icon.
Fix the initial value for expand/collapse
ensures collapsible xblocks have non-italicized expand/collapse toggles
added explanatory sidebar text to container page
updated container page sidebar label to reflect feedback from documentation team