Add linkchecker Systemd service and timer

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systemd/conservancy-www-linkchecker.service

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+# Check all internal links on the Conservancy website
+
+[Unit]
+Description=Check Conservancy website links
+
+[Service]
+Type=oneshot
+User=debian
+ExecStart=linkchecker https://sfconservancy.org/
+
+SystemCallFilter=~@clock @cpu-emulation @debug @module @mount @obsolete
+CapabilityBoundingSet=
+NoNewPrivileges=true
+
+PrivateDevices=true
+PrivateNetwork=false
+PrivateTmp=true
+PrivateUsers=false
+ProtectControlGroups=true
+ProtectHome=true
+ProtectKernelModules=true
+ProtectKernelTunables=true
+ProtectSystem=strict

systemd/conservancy-www-linkchecker.timer

@@ -0,0 +1,8 @@
+[Unit]
+Description=Check Conservancy website links
+
+[Timer]
+OnCalendar=weekly
+
+[Install]
+WantedBy=timers.target