Bug 28338: Make item-level holds use locally defined pickup branches

Bug 28338: Make item-level holds use locally defined pickup branches

Besides the commit subject, this patch does much more:
- It makes request.pl stop passing a pickup location to
  CanItemBeReserved
- It makes the page use the API to render a dropdown for each item, with
  their valid pickup locations
- Items with no valid pickup locations have a nice message about why
  they are disabled for selection

To test:
1. Apply this patch
2. Choose a biblio for placing a hold
3. Choose a patron
=> SUCCESS: You are presented with a new layout, that includes a
dropdown for choosing each item's pickup location. If an item is not
holdable, it still isn't.
4. Try having an item whose home branch is not marked as a pickup
   location
=> SUCCESS: Notice you cannot choose that item
5. CHoose an item, but do not choose a branch, and click 'Place hold'
=> SUCCESS: It shows an alert about the need to choose a pickup location
6. Choose one of the (only possible) pickup locations for the specific
   item
7. Place the item level hold
=> SUCCESS: All goes as expected!
8. Sign off :D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>