AccessDenied representation should have access to securitycontext

[jgou]

When using the representation builder to catch "access denied" exceptions set by the kauri-security filter, the concerning template can't use t:protect instructions because the securitycontext is already cleared at that stage.

A use case in which t:protect would be nice, is when you want to show the available options to the authenticated user on the "accessdenied" page.

related to #233