[PERF-303] Integer XBlocks/XModules into the static asset pipeline.
This PR, based on hackathon work from Christina/Andy, implements a way to discover all installed XBlocks and XModules and to enumerate their public assets, then pulling them in during the collectstatic phase and hashing them. In turn, the methods for generating URLs to resources will then returned the hashed name for assets, allowing them to be served from nginx/CDNs, and cached heavily.
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.
