Add ability to rebuild a course

Adds an entry to admin menu to allow the instructor to rebuild their course (issue #226).

This necessitated some changes to the way the course creation works. The `conf.py` and `index.rst` that are created when a new course is created are now saved in the directory `custom_courses/<coursename>/`. When the instructor wants to rebuild the course, these config files are used to do so.

Note: because of this change, the "rebuild course" functionality will *not* work on already existing courses (i.e. courses that have been created on the live site). I think this means that the instructor will have to re-create their course if they want the new layout/updates/etc when we do the upgrade. Let me know if you can think of a good way around this.