Add alertmanager and alert rules for prometheus #1307

Job	Run time
build-docker (internet.nl, docker/Dockerfile, app)	56s
build-docker (unbound, docker/Dockerfile, unbound)	57s
build-docker (linttest, docker/Dockerfile, linttest)	55s
build-docker (test-runner, docker/test-runner.Dockerfile)	1m 2s
build-docker (webserver, docker/webserver.Dockerfile)	54s
build-docker (grafana, docker/grafana.Dockerfile)	1m 4s
build-docker (cron, docker/cron.Dockerfile)	54s
docs	1s
integration-test	3m 12s
batch-integration-test	2m 39s
development-environment-test	2m 15s
lint/check	38s
test	2m 16s
	17m 43s

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