Unfortunately still prints a bold warning on deployment.

This will store the supposedly secret URL in the Nix Store and
print a honking big warning about it.

Only dashboards in this dir

... to be in sync with https://github.com/PrivateStorageio/PaymentServer/pull/98

Since !146, PaymentServer is proxied behind nginx, so there is no need to wait
for certificates or network interfaces before starting the server.

Write Python with Nix (?).

I hadn't understood: The PaymentServer turns 16 vouchers into
50 k ZKAPs.  So far, we don't have a metric for used up ZKAPs.

Also, since the dashboard is already a full browser page and
only handling payment, I rename the services dashboard to payment
dashboard.

Fixes privatestorageops#349.

... so it can be used in the nginx config instead of duplicating the number

As per @jcalderone and @florian videoconf on 2021-08-06.

- Remove custom handling of Let's Encrypt certs, use NixOs' ACME
  integration instead.

- Use NGINX for TLS termination.

Tryfix. Can't run this locally ATM.