Bug 14253: Acq - notify borrowers popup needs to allow scrolling

Bug 14253: Acq - notify borrowers popup needs to allow scrolling

The 'notify on receiving' patron search on the new order form
in acquisitions didn't allow you to scroll, so there was no
way to select users from the bottom of a longer result list.

To test:
- Create a new order in acquisitions
- On the order form, use the 'Add user' button to open
  the popup
- Perform a patron research with a lot of results
- Verify that with the patch you can scroll, but
  that you couldn't without it

Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
without patch: no scroll bar in Firefox 38
with patch: scrolling works fine

Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>