[Enrollments] Add a disclaimer or warning when adding a Teacher (or Teacher-based Role) to a course enrollment.

[ashley_hardesty]

By default, we do not allow Instructors (Teachers) to add Students to their course, since these course enrollments are populated by our SIS.

The problem is that the instructor can add other Roles to the course, such as Student:Observing, TA, and Teacher.  I'd like to see the ability to display a custom warning/alert when the Instructor tries to add another Teacher-based Role to the course.

What sometimes happens is that the Instructor needs to add a Student in a hurry, but when they go to +People, they don't see the Role of "Student" listed, so they choose "Teacher" instead (bad choice!).

I'd like to see something like: "The Role you selected will allow the user to have Full Control of the course, it's content and grades.  Is this what you want?" -- or something worded to let them know that this is not really a good choice.  There are times when we *do* allow the existing Instructor to add another Instructor (guest instructor, co-instructor, etc), so we still want to allow for this.