Bug 38097: Add class to not_retuned message

Bug 38097: Add class to not_retuned message

To test:
1: Enable ShowAllCheckins
2: Check in an item that was not checked out
3: Find "Item was not checked in" message in checkin table, confirm its only class is "problem"
4: apply patch, restart_all
5: repeat 2
6: confirm "Item was not checked in" now has the class "not_returned"

Signed-off-by: Eric Phetteplace <phette23@gmail.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>