Bug 12525: Prevent adding several patron lists with the same name

Bug 12525: Prevent adding several patron lists with the same name

If you add patron to a patron list, from the patron search result, a
list is created when you click on "Save".
The list is considered as new each time.

To reproduce:
1/ Launch a patron search
2/ Select 1 patron, and create a new list 'aaa'
3/ Select another patron and click Save again
2 lists are created

Test plan:
1/ Launch a patron search
2/ Select 1 patron, and create a new list 'aaa'
The dropdown list should be populated with this new list, and should be
selected
3/ Select another patron and click Save again
Only 1 list should be created

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>