Bug 18506: Add xShibOnly preference for Shibboleth authentication

Bug 18506: Add xShibOnly preference for Shibboleth authentication

This patch adds two system preferences to Koha, opacShibOnly and
staffShibOnly, allowing users to restrict authentication to just
one method, Shibboleth.

We do however, allow for local fallback for the SCO/SCI logins.

A system preference was chosen over a configuration file update to
allow for local override at the virtualhost level. In this way a
hosting provider can setup a 'backdoor opac' for example to allow
fallback to local logins for support operations.

Signed-off-by: Matthias Meusburger <matthias.meusburger@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>