Bug 34621: implement Patron import option to 'Renew existing patrons' 'from the curre...

Bug 34621: implement Patron import option to 'Renew existing patrons' 'from the current membership expiry date'

Test Plan:
1) Test importing a patron with the "from the current membership expiry date" option,
   note it does not work
2) Apply this patch
3) Restart all the things!
4) Re-test, note the expiration was renewed from the patron's current
   expiration date!

Signed-off-by: David Nind <david@davidnind.com>
Rebased-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>