Response time, probe fails and TLS expiry incl. alerts.

The Blackbox exporter can be used to check whether some services do answer;
We'll use it for our HTTPS endpoints.
Especially handy is it's checking for TLS cert expiry.

Fixes privatestorageops#358.

- The serivice can be reached by other machines.
- The metrics cannot be reached by other machines.

This changes the Throughput graph to be relative, changes its unit
to Bit/s, and fixes the labels of the Network pkt/s graph.

Fixes privatestorageops#24.

Fixes privatestorageops#355.

[Copy pasta](https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/blob/ca44204076983ce02fd3e0dacd940514e0821e23/nixos/modules/spending.nix#L130) - Thanks @tomprince!

Epiphany: Systemd reads more than a single environment file. This
lets me improve the file naming much.

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

We currently expose `ourpkgs` to nixos modules via an `ourpkgs` argument using
`_module.args`. However, `_module.args` is not available in the final
configuraiton. Thus, to allow tests to access the value, we also expose
is via passthru.

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 (?).