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.

Add Studio client-side runtime notifications for saving/cancelling XBlock edits.

LMS-1724

update course info module to save content in new 'items' field + update import/export to handle content field other than 'data' or 'metadata'
STUD-154

- Changed link URL of file name to use public/LMS url
- Added toggle link to display all public URLs for instructor convenience

STUD-1248

STUD-1248

This bug affected unpublishing content (and would trigger an error
message in studio). The fix was to uncouple the rendering of the studio_view
of an xblock with the student_view, so that content that wasn't in
draft mode already wouldn't be modified by early rendering of the
studio_view.

[STUD-1316]

STUD-967