OP-336 404 handling system and default view
Moving the setting of view and layout for the event to before the rendering of the not found viewlet

This means that the viewlet can override these values if it needs to
OP-336 Simpler convention for the 404 template configuration
OP-336 Making event.notFound() render our configured 'notFound' viewlet
OP-336 adding a custom route for /404.html, /500.html, etc. so that we can have tidy URLs that serve up configured error pages
OP-336 Documenting the 404 page system