Limit senders for email distribution lists

[daniel-lerch]

Currently, all distribution lists of Korga are public in terms of anyone can send an email to its alias which will be forwarded by Korga. This should be changed to an approach where only emails from authorized senders should be allowed. Authorization could happen via person filters and email addresses from ChurchTools. Korga should also verify that DMARC rules were followed to make sure the sender address has not been faked.