Bug 29407: Make the pickup locations dropdown JS reusable

Bug 29407: Make the pickup locations dropdown JS reusable

This patch generates a function to be used in places where select2
dropdowns are usesd for choosing pickup locations. This cleans
repeated/almost identical code introduced by different bugs.

To test:
1. Make sure choosing pickup locations works
2. Apply this patch
3. Repeat 1
=> SUCCESS: No functional changes
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>