Added a new column to user_students and user_teachers called "timeaccess"

Added a new column to user_students and user_teachers called "timeaccess"
for recording the last access to the COURSE.

This is updated at the same time as add_to_log and is now used instead
of user->lastaccess when course user listings are required.

This means course listings now show what you expect and open up the way
for a "current users" listing and instant messaging etc ...