Bug 17003: (follow-up) Update definitions according to voted RFC

Bug 17003: (follow-up) Update definitions according to voted RFC

Test plan:
1) Have some patrons with checkouts, some renewable and some not
renewable
2) Use your favorite API tester and access GET
http://koha.url/api/v1/checkouts/{checkout_id}/allows_renewal
3) Check the response is OK according to voted RFC:
https://wiki.koha-community.org/wiki/Checkouts_endpoint_RFC#Checkout_renewability_2
4) prove t/db_dependent/api/v1/checkouts.t

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Johanna Raisa <johanna.raisa@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>