Bug 13647: On-site checkouts - Allow library to specify due date

Bug 13647: On-site checkouts - Allow library to specify due date

This will remove the restriction that forces the due date for
an on-site checkout to the end of the current day.

It will still suggest the end of the day, but you will be able
to change the date to whatever you want using the datepicker
widget.

To test:

Before applying the patch:
- Turn on OnSiteCheckouts and OnSiteCheckoutsForce
  system preferences
- Check the on-site use checkbox and try to set due date
  manually, it should not be possible
- Check out the item and check the due date is set to the
  end of today
- Put a restriction on your patron account and repeat steps,
  the item checked out should again have a due date at the
  end of the day
> There should be no way to be able to check out an item
  with a 'custom' due date

Apply the patch:
- Repeat steps from above, check out
  - on-site use without a restriction on your account
    - without changing the due date
    - with a due date picked with the datepicker
  - on-site use with a restriction on your account
    ... see above
  - no on-site use but with a specified due date
  - no on-site use, not specifying a due date
- Turn off SpecifyDueDate
  - on-site use with and without restriction should
    set the due date automatically to the end of the date
    now, altering the due date should not be possible.

Signed-off-by: Nicole Engard <nengard@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>