* required some refactoring in create_account() view

* we appear to be fairly consistent in using is_staff and has_staff_access_to_course
* cleaned up some docstrings and little code things as I went
* fixed small bug in change_enrollment view (check for non-anon user)

* catch exceptions and return a 404 if course not found
* add Location.is_valid(), tests
* stub of jumpto/ view.

Django hardcodes the subject line for password reset emails. You're supposed
to be able to override it with a properly named template, but that didn't
make it into 1.3.1 (which is what we're deploying on now). We're not using
the Sites framework at this time, so this was the "fix".

Cleaned up activation pages. Added "Log In" link to notice when anonymous user tries to enroll in course.

If a user is anonymous when they click the enroll button, they will automatically be enrolled in the course if they immediately log in our create an account.

Registering now temporarily logs you in. A message is displayed on the dashboard, reminding you to activate your account. Logging in shows a reminder about activating.

Changed check_course from a decorator to a regular function that throws exceptions. Much easier to understand now.