merge develop into production

• use a different state directory for the issuer's voucher database (!226 (merged))
• Update nixpkgs. (!227 (merged), !223 (merged), !221 (merged), !219 (merged), !214 (merged), !207 (merged), !205 (merged), !204 (merged), !199 (merged), !193 (merged), !181 (merged))
• bump ZKAPAuthorizer to the fixed version (!225 (merged))
• Monitoring/tahoe metrics collector: Add systemd dependency declarations (!220 (merged))
• Update to version of ZKAPAuthorizer that includes a Tahoe with openmetrics support. (!218 (merged))
• Add dashboard for tahoe-lafs (!217 (merged))
• Monitoring: Add regular scraping for Tahoe OpenMetrics statistics (!216 (merged))
• Update privatestorage. (!208 (merged))
• add a rst version of our design template (!213 (merged))
• Fix https alerting (!210 (merged))
• Monitor MegaRAID on our storage nodes (!202 (merged))
• Update ops docs: No need for manually creating TLS keys for the payment server anymore (!212 (merged))
• words about recovering from backup (!206 (merged))
• Don't literalExample all the examples (!209 (merged))
• Monitor TLS cert expiry (!201 (merged))
• Update zkapauthorize to use mach-nix and nixos-21.05. (!166 (merged))
• Fix dev docs for repo update script (!203 (merged))
• Alerting: Only alert when an exporter is not scrapable for 5 mins or longer (!200 (merged))
• Use $__file{} for grafana's slack URL secret. (!191 (merged))
• Monitoring: Add dashboard and alerting for used swap (!198 (merged))
• Alerting: Don't when no scraping error ever happened. (!197 (merged))
• Use a version of vulnix that doesn't collapse derivations with different sets of patches. (!196 (merged))
• Update PaymentServer. (!195 (merged))
• Expose zkap-spending-service metrics. (!184 (merged))
• Monitoring: Add dash for scraping failures + alert (!188 (merged))
• Don't use our nixpkgs fork for LeaseReport. (!192 (merged))
• Protocol sequence diagrams for ZKAPs (!190 (merged))
• Monitoring dashboards: Rethink scales (!189 (merged))
• Add a nixos module for the ZKAP Spending Service (!172 (merged))
• Simplify local grid hardware (!182 (merged))
• Metric names: It's vouchers (plural) (!187 (merged))
• Make payment server answer with a 404 by default (!186 (merged))
• Enable alerting (!185 (merged))
• Use networking.fqdn to define targetHost, rather than constructing it ourselves. (!183 (merged))
• Set nixpkgs explicitly in morph. (!174 (merged))