Allow simultaneous enrollment in multiple groups with checkboxes.

Some of our clients needed the possibility that students be able to enroll into several groups simultaneously. This is my attempt to provide this functionality. It would be great if you could review the changes I've made and perhaps incorporate them into the main code so that we could always update from the main source tree.

Note: Some of the code is perhaps unnecessarily convoluted for the sake of keeping out of the way of the normal-behavior (so as to introduce the least amount of bugs possible).