Bug 28854: (follow-up) Replace unique constraint in return claims

Bug 28854: (follow-up) Replace unique constraint in return claims

This patch fixes the failing test for the return claims api.  We add a
new unique constraint to replace the one we removed earlier in the
patchset. One should not be able to create a duplicate return claim for
the same issue of an item.

Test plan
1) Run the database update
2) Confirm that t/db_dependent/api/v1/return_claims.t now passes
3) Confirm that t/db_dependent/api/v1/items.t continues to pass
4) Bonus points, confirm that bundle checkin still works when items are
   missing

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>