feat: support setup with only an IPv4 address, but no domain

[missytake]

replaces #894

This is a slightly alternative approach to ipv4-only relays. It splits up mail_domain in 3 different possible config values:

• mail_domain: can be the domain of a relay, or 13.12.23.42: this is used for routing
• mail_domain_deliverable: can be the domain of a relay, or [13.12.23.42]: this is used in the email addresses
• mail_domain_hostname: can be the domain of a relay, or 42.23.12.13.in-addr.arpa: this is used as myhostname in postfix

todo:

• fix tests
• look up every usage of mail_domain and see whether it's correctly used
• let cmdeploy dns fail gracefully
• make cmdeploy status work
• document experimental IPv4-only setups
  • explain that strict TLS & DKIM checking is disabled for them
• make CI test IPv4-only setups, e.g. with re-use cmlxc workflow, replacing CI with hetzner staging servers with local lxc containers #917

[j-g00da]

I'm not sure of benefits of this approach, do we need so much control over these?

[missytake]

I'm not sure of benefits of this approach, do we need so much control over these?

I think it's much less confusing in the long run, if we separate these different usages of mail_domain consistently. It only makes sense if there is IPv4-only CI though, so we can see if mail_domain_* is used wrongly somewhere.

[missytake]

The documentation linkchecker fails, but the internal link works (locally and in the staging documentation preview): https://staging.chatmail.at/doc/relay/919/getting_started.html

Any idea how we can link to pages in the same documentation in a way that the CI is happy?

edit: fixed