Mailcow-dockerized: add config option to when a cert is renewed

[Arlon1]

Is your feature request related to a problem? Please describe.
Let’s encrypt sends an email 20 days before a cert expires.

We recommend renewing certificates automatically when they have a third of their
total lifetime left. For Let's Encrypt's current 90-day certificates, that means
renewing 30 days before expiration.
-- https://letsencrypt.org/docs/integration-guide/

However mailcow renews certificates only 14 days before the expiry date.
Those 14 days (1209600 seconds) are hardcoded here:
https://github.com/mailcow/mailcow-dockerized/blob/9eebc7c524f46b1916b318b22bc2dbcdca607c41/data/Dockerfiles/acme/docker-entrypoint.sh#L431

Describe the solution you'd like
I suggest adding an option into a config file (mailcow.conf maybe?). It means the number of days a cert is renewed before its expiry.
This would add a little bit of convenience over editing the above mentioned file and changing the number of seconds.

[mkuron]

It probably makes more sense to change the 14 days to 30 days for everyone. This does not need to be configurable. Please submit a pull request.