WebSession: login_method changes

* Is more gentle when login_methods are altered on an existing
  installation (e.g. by falling back to the default login method
  if the settings of a user specify a no longer existing login_method).

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
WebAccess: new CERN auth method support

* Adds support for the CERN specific external_authentication_sso.py
  authentication plugin to expose more attributes provided by
  Shibboleth upon authentication, including the authentication method
  used by the user and the user identity class.

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.0' into maint-1.1

* maint-1.0:
  WebSession: login_method changes
Merge branch 'maint-1.1'

* maint-1.1:
  WebSession: login_method changes