Bug 20468: (follow-up) Cancel multiple items for same reason

Bug 20468: (follow-up) Cancel multiple items for same reason

The repeated prompts for cancelling multiple selected items are
confusing.
The wording is slightly adjusted. The reason is asked only once per
group of selected items.

Test plan:
Create three requests.
Select two requests and cancel (from top menu) for reason A.
Cancel third request (from item menu) for reason B.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>