... 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.

NGINX does longest-prefix-match for selecting locations.

Tryfix. Can't run this locally ATM.

System tests do currently not include testing our monitoringvpn wireguard
setup, so let's just add the required IP by an alias to the lo interface.

This should fail currently, since metrics are still public.
Also, I couldn't test this code locally, since I currently run VirtualBox VMs
on my dev box. I'll just see what CI says to this code.

Add a timer service to periodically trigger the cert renewal service

Closes privatestorageops#222

See merge request !143

This causes NixOS to generate the systemd timer unit for us and saves us
having to explain a lot of subtle systemd features.

It does pin execution to midnight on Monday but scaling to handle load spikes
is Let's Encrypt's problem, I guess.

Add ssh.nix to monitoring systems

See merge request !142

Monitoring: Set admin PW

See merge request !123

Fix deployment key whitespace

Closes #77

See merge request !138

This makes it easier to test outside of GitLab and it also means we don't
depend on whatever wacko shell settings we inherit from GitLab.

Use a new variable which actually holds a path

Closes #76

See merge request !136

Remove group and other permissions from the deploy key

Closes #75

See merge request !134

Propagate morph success/failure out of update-deployment

Closes #74

See merge request !132

Burn in some SSH host key knowledge

Closes #71

See merge request !130

Bounded NIX_PATH

Closes #72

See merge request !131

We can't point it at a Nix expression that evaluates to our desired version of
nixpkgs because certain consumers want it to be a filesystem path instead.

This is what we wanted and much more directly.  It also avoids the gradual
lengthening of the path by always setting the path to nixpkgs-2105 instead of
some derivative of whatever the path was last time.