Improve Course Duplication detection

[philipmjohnson]

Please do work for this task in a branch called issue-708.

Currently, it's pretty easy to define a course twice. Please update the add course method to check that neither the course name nor course number fields are already defined when creating a new course.

[cammoore]

I suggest we remove the slug and num fields from the form and replace them with department and course number fields. We can then construct the course slug and course num values.