fixes #22
This commit introduces:
- A few more optional options for the monitoring module, in particular
an SMTP option to setup sending alerts with an STMP server.
- 2 required options for adding a secure key for signing and for an
initial admin password. The latter is nice because at least you can
choose securely the initial admin password instead of it being just
"admin", adding a bit more security to the install process.
- Provisioning Grafana with dashboards, datasources, alerts, contact
points and notification policies.
- Documentation for monitoring in
[docs/blocks/monitoring.md](docs/blocks/monitoring.md).
- A NixOS test that makes sure provisioning did go well as expected.
