Add management command to Studio for exporting all courses

Studio currently keeps no history of courses authored in the system.  This is an issue for researchers studying course data, as well as for course teams who may need to look back to see what changes were made, or who need to recover earlier revisions.

This PR adds a management script to allow all courses stored in the mongo db to be exported to a directory.

At MIT, we're using this with a cronjob and a git repository, to regularly take snapshots of all Studio course content.