Bug 28273: Multi-hold should not offer invalid pickup locations

Bug 28273: Multi-hold should not offer invalid pickup locations

This patch makes the multi-hold page offer only valid pickup locations
for the selected biblios. Prior to this, all system-wide pickup
locations were offered.

To test:
1. Set 'Hold pickup library match' to 'Item's home branch' so we put a
   constraint on the valid pickup locations for easier testing.
2. Choose two or more biblios from a search, which contain  in total 2
   or 3 item home branches.
3. Click 'Place hold'
4. Choose a patron
=> FAIL: The dropdown offers all system's pickup locations
5. Apply this patches
6. Reload the page
=> SUCCESS: Only valid pickup locations are offered
7. Sign off :-D

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>