- Only redirect if the appropriate flag is set on the Domain
  (default is not to redirect)
- Use a 302 redirect for now. Ideally this is a 301 redirect,
  but redirecting other people's domains with a 301 is always a bit
  risky.

- Do HTTP -> HTTPS redirects if domain.https is set
- Redirect public domain to canonical CNAME if domain.canonical
- Redirect CNAME -> public domain if the custom domain is not canonical
- Set additional data on the X-RTD-Redirect header whether the redirect
  is to canonicalize, a system redirect, or a user redirect