Handle taking registrations on a course with TBD start date

Works by setting start date > a magic constant set in LMS settings
and by setting enrollment_start < now.

At Stanford we sometimes would like to advertise a course publicly before committing to a start date.  This small patch allows us to do that by setting start date to some time way in the future.

I thought a magic constant in LMS settings was better than a magic constant in course_module.py, so that's why I didn't write a course.start_date_is_determined() function there.  The change seems smaller this way.