Make descriptorsystem inherit from Runtime
Switch to studio_view
Fix repeated doc builds
Make builds quiet by default
Refactor wrap_xmodule call
Fix notification problem
Add verbose option
Fix import mocks.

    Prevents sphinx from bailing on builds that involve unavailable imports.
Add in and route control options
Clean up item views, use JsonResponse class
improving code style
Refactored get_module_previews function
XBlock integration: replaced `get_html` with `runtime.render()`

Currently calls the same machinery, but re-routes the logic in preparation of
deeper integration with XBlock
Made some tests more general, less fragile
Make sure to return the content, not the fragment
Docstrings
Fix defaults
Removed get_module_previews function

According to @cpennington, no modules return anything for `get_sample_state`,
so this function is extraneous.
Update docstring
Fix tests