[MICROBA-1178]
- remove modulestore usage in `generation_handler.py`
- add duplicate functions that utilize a CourseKey or CourseOverview to remove dependence on modulestore (this will be cleaned up (if possible) at a later part of this refactor)
- add python API function to `content`/`course_overview` app that will retrieve a single CourseOverview (rather than a serialized list of dicts of CourseOverview data)