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
Merge pull request #564 from edx/db/xblock-student-view

XBlock integration for Studio: student view