Start of studio support for libraries: two views, nav menu link, urlconf, and feature flag
A busted read-only view of library content, based on container template

Shows the XBlock children stored in the library - just don't press any buttons
Include library version info in the JSON API view
Stop Studio REST API methods from stripping branch data.

Most edit actions in studio now work and create a versioned history.
Fix deleting blocks in studio: split mongo's draft awareness was overriding the branch