Grady/authoring/rubric gui

Very Close (not complete front end work, and some deletion JS needs to be tweaked.  Other than that, it is ready for code review!  Going to be done tomorrow!
Right -- assuming we have test coverage for the validation logic, we can get away with two integration tests at the XBlock level: one with a valid definition and one with an invalid definition.

But please make sure we do have comprehensive test coverage of the validation code.  This means more than simply line coverage: the test cases need to cover:
* Missing keys / extra keys
* Keys with unexpected types (`None`, int, non-ASCII unicode)