[Improvement] Streamline .well-known endpoints

We currently set up .well-known endpoints in different places. The /client endpoint is controlled through the well_known_client option, whereas the /server endpoint is configured in the reverse proxy (!531 (closed)) and requires users to fill in base_url.

Possible improvements:

1. Add a well_known_server_route, to be configured similarly via config option.
2. Since both well_known_client and base_url are based on server_name, we can potentially hide these options from the user and set both by prepending https:// to server_name (given proxy is already supposed to get TLS).

Any drawbacks that I’m missing here?