Bug 35942: OPAC user can enroll several times to the same club

Bug 35942: OPAC user can enroll several times to the same club

Test Plan:

1) Create 3 clubs, 1 limited to library A, 1 limited to library B and one not limited
2) Use a patron with home library A.
3) Go to the opac-user page, "Clubs" tab show 0/2 (the one from library B is not listed)
4) Browse to /cgi-bin/koha/svc/club/enroll?id=1
5) Reload that page a couple times
6) Note the patron is now enrolled in the same club multiple times
7) Delete those enrollments
8) Apply this patch
9) Restart all the things!
10) Repeat steps 2-7, note the lack of duplicate enrollments!
11) Repeat steps 2-10 for the staff interface

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>