[API] Add ability to "force" an enrollment through API on closed courses.

Problem statement:

Currently we cannot add enrollments to concluded, frozen, out of date, etc., courses. Occasionally we need to do this in order to provide access to source material to new instructors or such. In order to accomplish this we need to manually create an upload an enrollment SIS file.

Proposed solution:

Adding a "force" flag to this endpoint: https://canvas.instructure.com/doc/api/enrollments.html#method.enrollments_api.create ...would allow us to push this through using our existing integrations and automation structure. I'm sure others would find it helpful as well.

User role(s):

admin