Fix student search

This is fixed and scoped, as described in our email.

Might want to consider changing this search functionality to be AJAX-ified in the future, so that we're not loading a list of all students in the current user's courses on every page load for a feature they may not even be using, and to prevent loading a list of all students into the page source.

Also, wasn't sure if we should maybe scope the student search to just the current course, instead of all of the current user's courses, since the course select changes the scope of everything else on the page already.