Bug 29002: Assign item at time of booking

Bug 29002: Assign item at time of booking

This patch updates the logic to ensure we always assign an item at the
point of booking instead of assuming we can assign the item at fulfilment time.

This makes the logic significantly simpler on the client when trying to
calculate available dates.

Test plan
1) Attempt to create a new booking using the 'Any item' option in the
   modal
2) Note that upon placing your booking you are assigned an item for that
   booking.

NOTE: We add a FIXME to the client code to highlight something to test
when/if someone decides to have another try and moving the item
assigment to just in time as aposed to up front.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Janet McGowan <janet.mcgowan@ptfs-europe.com>
Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Laurence Rault <laurence.rault@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>