Dropping the use of MOODLE_PAGE_COURSE <gasp>!

The new constant is PAGE_COURSE_VIEW, and it's more streamlined with the
new CSS-related ids for 1.5. There are great possibilities for the future
here, so we 're getting ready.

On the other hand, this has to be the ugliest hack I 've ever inflicted
on any program (look at pagelib.php). Thankfully it will only stay for just
a little while.