provision grafana datasources and some dashboards

[ibizaman]

fixes #22

This PR 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.
• A NixOS test that makes sure provisioning did go well as expected.