This patch hopefully fixes issues with sending kill signals to subprocesses from the master process of dovecot.

Exmaple error logs:
```
Jan 25 00:27:16 master: Error: service(auth): kill(14, SIGINT) failed: Operation not permitted
Jan 25 00:28:16 master: Error: service(auth): Process 14 is ignoring idle SIGINT
```

This patch moves the dovecot config from the container build to the helm
chart which allows to use templating with values that are configured at
runtime. This should provide more flexibility and standardizes the
dovecot container further.

Given that currently a lot of connections look broken and result quite delayed
mail delivery adjusting this interval will hopefully help to keep imap
connections alive and therefore delivering emails in realtime.

Currently for some reason postfix decided to drop all email that are
supposed to be relayed by default. This is obviously unwanted behaviour.
This patch takes the settings from the workaround.org project, applies
them to submission and submissions, which fixes the current issue.

However I still don't fully understand why.

References:
https://workaround.org/ispmail/bullseye/relay-outoing-email-through-postfix/

This patch switches the externalTrafficPolicy to `Local` for the
proxy-protocol enabled services, in order to skip kube-proxy when
connecting, this makes sure that proxy-protocol can actually function
and doesn't hit a node that would need to redirect traffic.

References:
https://metallb.org/usage/#layer2
https://github.com/kubernetes/kubernetes/issues/66607
https://github.com/digitalocean/digitalocean-cloud-controller-manager/pull/206

This patch disables error queries from zenhaus. It requires some more
tinkering down the line, but should accept email for now.

This patch allows to set the `HOSTNAME` variable, which should
explicitly set the hostname for the postfix pod and configure it to use
the proper external DNS name.

DNSWL went subscriber-only apparently.

References:
https://www.dnswl.org/

This patch deployes the fixed postfix image containg postscreen with
enabled smtpd.

This patch creates a PDB for postfix, if the setup is scaled to more
than 1 postfix replica. This way at least one postfix instance should
always be available and the MTA therefore HA.

Currently the dovecot backend is not yet HA, this should take place
later on.

Currently postfix reports and error due to the missing service. This
patch fixes the problem by enabling the expected service in master.cf

This patch should enable the postfix setup in MoK to utilise the
proxy-protocol and therefore gather required IP information from
connecting clients.

It's implemented based on some documentation regard the topic and is
useful for reverse-proxy setups e.g. behind haproxy.

References:
https://www.haproxy.com/blog/efficient-smtp-relay-infrastructure-with-postfix-and-load-balancers/

Currently the service name for dovecot was hardcoded in the image, which
makes it incompatible with MoK. As a result this patch updates the image
to allow a dynamic overwrite on startup as well as updating MoK to
account for the new functionality.

This patch adds the needed network policies to the helm chart. This
provides a reasonable base-line setup. Currently any customizations
regarding network access would require additional policies. These might
come in place later on.

Adding a new helm chart to deploy functional mail server on Kubernetes.
It's composed of postfix and dovecot so far and handles user management
using Kubernetes secrets.