This patch reworks how the infrastructure setup is organised to provide
more flexibility when maintaining multiple clusters with the same
repository as well as resolving various bootstrapping issues.

The new structure will move various overwrites into the cluster-specific
infrastructure directory, mimicking the apps directory. Currently it's
quite split between infrastructure and the cluster directory.

This patch refactors the entire ingress setup into two ingress classes:
internet and intranet. This allows to ensure that they use separate
services, reduces the attack surface and enables the usage of
external-dns.

It turned out, that pushing all warning to GitLab resulted in being too
noisy. There were more than 30 issues opened in no time, often
duplicates, due to how GitLab maps or rather doesn't map alerts in the
free version.

This reverts commit de6e02d9.

This patch enables the delivery of alerts with the severity warning to
GitLab, which allows to handle the issues without causing alert fatique.