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

[LMS-230] [LMS-229]

I tried to make DictFieldData strict about only being used with one
block.  That effort failed ultimately, but these are reasonable changes
I made along the way.

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.