Bug 28947: Prevent OPAC user to create new users

Bug 28947: Prevent OPAC user to create new users

This patch prevents an existing user from exploiting the patron edit form in order to
force create new patrons

To test:
Try all combinations of PatronSelfRegistration and PatronSelfRegistrationVerifyByEmail
with and without this patch.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
(cherry picked from commit eab5f18787d5b7b5efc374a7dc87ef4090e5bac0)
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>