We've had it on `matrix-nginx-proxy` before, but
our initial support for Traefik did not include any of these security
headers.

Previously, we had to run it at a subpath, like `/synapse-admin`.

We can now dedicate a whole domain and the `/` path to it, should we
wish to do so.

Previously, it had to go through matrix-nginx-proxy.
It's exposed to Traefik directly via container labels now.