MT8122: Adds Shibboleth authentication

 - Use the shibbolethAuthentication syspref to enable Shibboleth authentication
 - Configure the shibbolethLoginAttribute to specify which shibboleth user
   attribute matches the koha login
 - Make sure the OPACBaseURL is correctly set
MT8122, Follow-up: Adds Shibboleth authentication

  - Fix logout bug: shibboleth logout now occurs only when
    the session is a shibboleth one.
  - Do some refactoring: getting shibboleth username is now
    done in C4::Auth_with_Shibboleth.pm (get_login_shib function)
MT8122, Follow-up: Adds Shibboleth authentication

  - Adds redirect to opac after logout
MT8122, Follow-up: Adds Shibboleth authentication

 - Shibboleth is not compatible with basic http authentication
 in C4/Auth.pm. This patch fixes that.
MT8122, Follow-up: Adds Shibboleth authentication

 - Use ENV{'SERVER_NAME'} instead of syspref OpacBaseURL in order to work with
 multiple vhosts.
MT8122, Follow-up: Adds Shibboleth authentication

  - Adds missing protocol for $ENV{'SERVER_NAME'}