Bug 18298: Move password generation to template side

Bug 18298: Move password generation to template side

This patch removes a really ugly way to generate a password: the whole
template was sent and parsed to retrieve the "#defaultnewpassfield" node.
To avoid the password to be sent plain text it is certainly better to
generate it client-side.
The same kind of passwords will be generated: 0-9a-zA-Z
The while loop prevents to get an invalid generated password.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>