Bug 22630: Allow to change homebranch in course reserves

Bug 22630: Allow to change homebranch in course reserves

Test plan:
1. Create a course (disabled)
2. Add a reserve to this course for an item and set a homebranch
   different from the item's homebranch
3. Enable the course
4. Verify that the item's homebranch has changed
5. Disable the course
6. Verify that the item's homebranch was reset to its initial value
7. prove t/db_dependent/CourseReserves/CourseItems.t

Sponsored-by: Université de Lyon 3
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Sonia Bouis <sonia.bouis@univ-lyon3.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>