Merge (mostly) hidden config options and their uses from proko2, namely:

- the ability to have a particular domain only allowed for MDNs
- the ability to send no MDNs to SPAM
- the ability to configure access to the above two options
- extended sieve parsing for vacation settings
- the ability to strip certain mimetypes (such as invitations) when inline forwarding emails automatically

These are mostly useful for Kiosk.