Multiuser loadtest

This is dependent on #599 (robust userlogs), as that's what I'm testing.
Four small features to make this more useful to me:
* Select the login from amongst N users (configurable via querystring)
* Create users, if necessary (based on N)
* Ability to logout (important for UserLog testing), based on configurable probability (via querystring)
* Configure how likely to do an exercise or video (via querystring)