Add a nixos module for the ZKAP Spending Service

See merge request !172

Simplify local grid hardware

See merge request !182

Metric names: It's vouchers (plural)

See merge request !187

Make payment server answer with a 404 by default

See merge request !186

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

Enable alerting

Closes privatestorageops#392

See merge request !185

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.

Use `networking.fqdn` to define `targetHost`, rather than constructing it ourselves.

See merge request !183

Update nixpkgs.

See merge request !181

Set nixpkgs explicitly in morph.

Closes #85

See merge request !174

Only dashboards in this dir

It looks like this may have been originally copied from a newer nixpkgs
since it wasn't in our nixpkgs fork.

The differences I can see:
- Details of `sudoer` file, some of which appear to be duplicates.
- `jq` and `gnumake` are not on the system path
- `vagrant` user is in group `users` instead of `vagrant`.

Update payment metric names

See merge request !179

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

Add a script to update nixos version, and run it.

Closes #83

See merge request !165

upgrade PaymentServer

See merge request !178

continuously deploy develop to staging

Closes #82

See merge request !175

Change the on-grid continuous deployment tool to deploy the "develop" branch to the staging grid

See merge request !176