Pull XModule attributes out into a mixin that can be applied to xblocks
Switch get_html replacement out for using wrap_child
Add a default studio view
Use XBlock rather than XModuleDescriptor for access checks
Make XModuleDescriptor and XModule act as a single class

By transparently proxying between the XModuleDescriptor and the XModule,
and between their runtimes, we can make them act as a single class, so
that we can swap in an actual XBlock instead.
Use XBlocks instead of XModules when loading