Use XBlock handlers for handle_ajax in XModules

Adds xblock handler_url support to the LMS, and makes handle_ajax use
that code.

[LMS-230] [LMS-229]
Update handler_url support to match the latest in the xblock repo, for both js and python runtimes
Use XBlock rather than XModuleDescriptor for access checks
Use XBlocks instead of XModules when loading
Allow use of any XBlock installed as an advanced component in Studio
Add a default studio view with instructions on how to edit an xxblock directly in mongo