host static files for emails on cloudfront
host static files for emails on cloudfront

emails are read from all over the place, and each one will load the
gov.uk crest and optional dept branding images. rather than have
those hosted on www (which is routed through to the admin flask app),
host them on static.notifications.services.gov.uk, which links to
cloudfront (on all environments)

alphagov/notifications-utils