Branding: move AAF mentions from GSP to messages

    Branding: move AAF mentions from GSP to messages
    
    Extract AAF specific mentions out of GSP code under
    app/federationregistry/grails-app/views (403.gsp, 404.gsp and 500.gsp)
    and move the text into messages-branding.properties.
    
    Notice: some of the text includes HTML formatting and is rendered
    WITHOUT the HTML encoder.