This allows XModules (specifically CombinedOpenEnded) to use ajax_url
during their init functions (which would, before, have thrown an
exception).
[LMS-1493]
This required changing structural XModules to fully implement
student_view, rather than just returning the HTML of their children in a
get_html call.
[LMS-223]
[LMS-1170]
stores_state is now only used in a single test, in conjunction with
has_score. In practice, it's never the case that stores_state is false
when has_score is true, so we can delete stores_state entirely, and just
use has_score for the grading test.
