Bug 30802: Make numReturnedItemsToShow function properly

Bug 30802: Make numReturnedItemsToShow function properly

Replaced the hardcoded 20 by the value of numReturnedItemsToShow to
display the right number of items in check in.

Test plan:
1-  Make lots of checkouts, at least like 40 (I used the batchCheckouts feature)
2-  Go to Circulation > Check in
3-  Return 21 items
    The last 20 items returned will be displayed
4-  Change numReturnedItemsToShow to 50
5-  Return a couple more items
    Only the last 20 returned items are displayed
6-  Change numReturnedItemsToShow to 10
7-  Return a couple more items
    Only the last 10 returned items are displayed
8-  Apply the patch
9-  Change numReturnedItemsToShow back to 20
10- Do steps 1 to 7 again, but this time step 5 returns the right amount
of items

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>