Bug 28762: Use Koha::Course in course-details controller

Bug 28762: Use Koha::Course in course-details controller

This patch updates Koha::Course to include the 'instructors' relation
accessor and then update the course-details controller to use the
Koha::Course object and pass it to the template instead of building a
hash using GetCourse.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 7f38c77d091cd4192b039041bcc3a00a05e10583)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>