Draft: Feature/rapid cdn custom error pages

Design proposal

The CDN shall expose URL, or list of it, to which the operator can upload the HAProxy friendly webpage (see software/rapid-cdn/templates/notfound.html for an example) for each error code. It'll overwrite default behavior. Doing delete to this endpoint shall reset the web page to default one. For case of this MR only 404 might be implemented, as improving more codes might require more work, including more descriptive default pages.