Bug 24159: Use days_mode circ rule to calculate the dropbox date

Bug 24159: Use days_mode circ rule to calculate the dropbox date

Calculate the drop box date depending on the circ rule. Note that only
the branchcode is taken into account here. Is that correct?

Test plan:
Use the drop box date feature and confirm that it takes into account the
calendar, depending on the circ rule.

Signed-off-by: Simon Perry <simon.perry@itcarlow.ie>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>