Manage access to payment metrics

Closes privatestorageops#368

See merge request !146

Set `NIX_PATH` in `shellHook` of `shell.nix`.

See merge request !162

If `nix-shell` is run interactively on NixOS, in way that has loaded
`/etc/profile` (for example `docker exec <container> nix-shell`), then
`/etc/profile` will be sourced in the shell that nix-shell starts. That
overwrites the `NIX_PATH` set by `shell.nix`.

If we instead set `NIX_PATH` in the the `shellHook`, that always takes
precedence.

Make the per-grid inline module more explicit.

See merge request privatestorage/PrivateStorageio!160

Adopt the Nix packaging from our nixpkgs fork and use it

Closes #79

See merge request privatestorage/PrivateStorageio!158

Merge remote-tracking branch 'whetstone/368.manage-access-to-payment-metrics' into 368.manage-access-to-payment-metrics

Update issue link.

See merge request privatestorage/PrivateStorageio!155

Access morph module options via config.

See merge request privatestorage/PrivateStorageio!156

Upgrade system test software and port system tests from Perl to Python

Closes #41

See merge request privatestorage/PrivateStorageio!151

This reduces the confusion with the nixos/modules/deployment.nix module,
and should make it clearer that it can contain any configuration that
applies to all hosts on the grid.

Write Python with Nix (?).

This lets other modules override these settings.

Services Dashboard: Payments

Closes privatestorageops#290 and privatestorageops#349

See merge request privatestorage/PrivateStorageio!154

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.

Clean up nix-store on the machines with limited disk space

See merge request privatestorage/PrivateStorageio!149

Less swap on disk constrained AWS hosts

See merge request !150