imfreedom/ansible

Remove some old dependencies and move the mercurial dep to the role
- https://issues.imfreedom.org/issue/INFRASTRUCTURE-16?project=INFRASTRUCTURE
- Postfix
- auth to Dovecot
- Dovecot
- Auth to Hub using:
https://wiki2.dovecot.org/PasswordDatabase/oauth2
otherwise fallback to trying this:
https://keep.imfreedom.org/grim/prosody_mod_auth_jetbrains_hub/
- IMAP, no POP
- Maildirs (in a vmail setup, no Unix users)
- No sieve for now
- SpamAssassin
- How to integrate?
- Mailman
- Importing archives is important