Add alertmanager and alert rules for prometheus #1303

Job	Run time
build-docker (internet.nl, docker/Dockerfile, app)	1m 19s
build-docker (unbound, docker/Dockerfile, unbound)	1m 8s
build-docker (linttest, docker/Dockerfile, linttest)	1m 19s
build-docker (test-runner, docker/test-runner.Dockerfile)	56s
build-docker (webserver, docker/webserver.Dockerfile)	1m 19s
build-docker (grafana, docker/grafana.Dockerfile)	1m 6s
build-docker (cron, docker/cron.Dockerfile)	1m 2s
batch-integration-test	2m 30s
development-environment-test	3m 1s
docs	1s
integration-test	3m 19s
lint/check	1m 1s
test	2m 22s
	20m 23s

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