Make descriptorsystem inherit from Runtime
Switch to studio_view
Refactor wrap_xmodule call
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
Removed get_module_previews function

According to @cpennington, no modules return anything for `get_sample_state`,
so this function is extraneous.
Update docstring
Change console logging stream from stdout to stderr
Fix manage.py to ouput the help of the django command if requested

Commands like the following were not working correctly:
```
$ python manage.py lms runserver --lms
```
Updated tests to weaken "number" input field requirement, which
isn't supported in Firefox.
add privilege copy
hide empty management list selector, add explanation text
enable beta dashboard
Migrate video tests to videoalpha tests, remove video tests.
Remove js unit tests.