Adjust parameters to construct_xblock_from_class to match XBlock repo.
Adapt to latest XBlock change: student_id -> user_id
Minor cosmetic code fixes.
Use XModuleDescriptor.parse_xml to interpret XML.
Add a management command to dump xml courses

This command loads the courses with the XMLModuleStore, and then exports
the fields contents as json in a diff-friendly way. This can then be
used to validate against regressions in the xml parsing code.