[feat] Explicitly indicate the catch-all or default forwarding rule in DNS TXT records

[schultzter]

Describe the feature

As per #264 and 161 the naked forwarding rules (Option A & E in the FAQ) are catch-alls after the named forwarding rules are applied (all the other Options). This should be explicit so that it's self-documenting and obvious whenever you look at it.

I would suggest the value of that TXT record begin with forward-email-catchall= and/or forward-email-default= to indicate that is the rule that will be used if none of the others match.

Thanks

Checklist

• I have searched through GitHub issues for similar issues.
• I have completely read through the README and documentation.

[schultzter]

NOTE: on the backend it already seems to work this way so this is just labelling on the front-end.