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>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>