Split modulestore persists data in three MongoDB "collections": course_index (list of courses and the current version of each), structure (outline of the courses, and some XBlock fields), and definition (other XBlock fields). While "structure" and "definition" data can get very large, which is one of the reasons MongoDB was chosen for modulestore, the course index data is very small.
This commit starts writing course indexes (active_versions) to both MySQL and Mongo, but continues to read from MongoDB only.
By moving course index data to MySQL / a django model, we get these advantages:
* Full history of changes to the course index data is now preserved
* Includes a django admin view to inspect the list of courses and libraries
* It's much easier to "reset" a corrupted course to a known working state, by using the simple-history revert tools from the django admin.
* The remaining MongoDB collections (structure and definition) are essentially just used as key-value stores of large JSON data structures. This paves the way for future changes that allow migrating courses one at a time from MongoDB to S3, and thus eliminating any use of MongoDB by split modulestore, simplifying the stack.
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