Some changes to user database functions:

Some changes to user database functions:

1) get_course_students, get_course_users and count_course_students when called with course=site will now use get_site_users instead of get_users. This I believe was the consensus reached in the discussions of how front-page activities should behave.

2) all functions can handle a list of exceptions now.

3) get_site_users now returns users in the order admins, teachers, students. Similarly for get_course_users. This makes the sorting bug 1727 a bit more bearable

4) new function search_users

The whole thing is really a mess because each function has slightly different conventions for its arguments. But the beta is too close to tidy this up now.