Add tests of opaque keys
Add definition_key to UsageKey
reorder and rename args for functions in access.py
Refactor to opaque keys
Remove Location() creation from LMS non-test files
Remove Location instantiations from CMS non-test files
Remove instantiation of Location() from common
Move more course_id/location conversions over
make export_to_xml take a course_id
Make delete_course take a course_id
Fix some incorrect calls to locators.
WIP
Implementation of CourseLocation keys
More fixes to has_access switching
First step towards updating asset functions to use AssetKeys
Remove nearly all uses of CourseDescriptor.id_to_location
Consolidate uses of course_from_id
Rename get_item_errors to get_course_errors
Fix uses of load_test_import_course
Turn OpaqueKeys single-value methods to properties