Course manager/owner cannot fetch course information from other instances when selecting source course (404) returned

[cysjonathan]

Revisiting item 2 in #4527 because a cross-instance instructor flagged this issue out

Issue

Course manager/owner cannot correctly fetch course information from other instancewhen "selecting" source course.

Current behaviour

If I am in instance this.coursemology.org, and I want to select another course 321 (in other.corusemology.org) to duplicate from, the dropdown selection will fail with a 404 as it is looking for
https://this.coursemology.org/courses/321 instead of
https://other.coursemology.org/courses/321.

Possible fixes

1. Reserve the course duplication page to only show the current source course, and disallow selecting other source courses.
   If user wishes to duplicate data from other courses, they should navigate to that associated course.

2. On selection of some other course, redirect user to that course's duplication page instead.

3. Move the duplication page outside of the course level, to the instance level, update the fetch calls to reflect the correct subdomains and allow for cross-subdomain logins.