Bug 36175: Corrections to reduced due for booked items

Bug 36175: Corrections to reduced due for booked items

This patch fixes a mistake in the circulation controller such that the
proper reduceddue date is passed to the template when a checkout would
overlap with a booking.

I also take the oportunity to enhance the functionality to allow
librarians to set their own reduceddue date between now and the hard
reduceddue as required for the existing booking.

Test plan
1) Set an item to 'bookable'
2) Add a booking to that item for user A for a few days time.
3) Attempt to checkout the item to user B and note the warning that
   the item is booked for another user.
4) With the patch applied, a reduced due date of 1 day prior to the
   booking should be defaulted to in the displayed date picker.
5) You should be able to select any date between today and the day
   before the booking starts
6) Confirm you can checkout with the reduceddue date set.
7) Bonus point, confirm the field is required, so if you try to wipe it
   and then checkout, you will be blocked.

Signed-off-by: Kelly <kelly@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>