Bug 27016: Make the pickup locations dropdowns use Select2

Bug 27016: Make the pickup locations dropdowns use Select2

On the shoulders of bug 27015, this patch makes the pickup locations
dropdowns use Select2 to handle both fetching the pickup location
on-deman through the API, and also searching for them.

This provides a better user experience for really big lists.

To test:
1. Have some holds on a biblio
=> SUCCESS: Bug 26988 works, made it load the data when you click on the
dropdowns.
2. Apply this patch
3. Reload the page
=> SUCCESS: Similar-ish behaviour
=> SUCCESS: Notice you can search for branch names and make the list
shrink
4. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>