[Enrollments] Allow all of a teacher's enrollments to be concluded at once

[e_j_armstrong]

Problem statement:

Sometimes staff leave or have something more unfortunate happen. We don't want to delete their account (as we need to keep a history of marking etc.) but often they are enrolled as a teacher on a large number of courses and at the moment we have to conclude each one individually

Proposed solution:

The ability to select all a user's courses and conclude them.

User role(s):

admin