Added add/remove course functions

Tried implementing locks/isolation, but ran into a lot of trouble. Right
now you can add/remove courses, but the handler file doesn't redirect
properly and I'm not sure how to fix that.

I also realized near the end that I'll need to look into making sure
students can't sign up for two of the same courses with different
sections on the database side. Also need to modify the current triggers
so the available seats can't go below zero.