Puts course listings in a sane order

@cpennington @nedbat Right now, if I go to edx.org, and look at the courses I'm taking, I have to scroll through 6 pages of things I signed up for but never touched. This is a supremely bad user experience. I'm sure I did a ton of things wrong, but this code applies a heuristic where courses are ordered on the page in roughly the order in which I last visited them (defined as "clicked around in courseware a little bit"). Recent courses are on top. Old courses are on bottom. I can go to courses I'm active in quickly and immediately.