STUD-1465
Changed the XBlock rendering logic to use promises to track asynchronous tasks.
Each dependency is then only loaded once the previous one has completed, and the
XBlock initialization only happens once all dependencies are loaded. This gives
XBlocks the same semantics in Studio as they get when shown directly on a page.
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.
