This MR extends the existing mail-server SR to a full mail server bundle, including:

• a user database (including aliases) backing both Postfix and Dovecot, that can be populated using shared instances (previously using Postgres, now using plain text per JP request)
• a preconfigured Snappymail webmail (using the apache-php component)
• the configuration needed to interact with the outside world

The last point revolves around two facts:

• that we can't assume the machine hosting the instance to have a public IPv4 address, which is unfortunately still a requirement for receiving and sending mail since numerous providers can't handle IPv6 yet.
• even if the machine had an IPv4 address, we can't rely on it to send mail: most providers use proprietary IP range lists for spam management, so reliable mail posting must be done through some kind of trusted relay. This can be done using third-party services such as Scaleway, Sendinblue or Rapidmail. Currently, the testing is done using an OVH VPS -- that's enough for a few mails.

Currently:

• relay configuration is hard-coded. In the end, this will be automated.
• network components (Postfix, Dovecot, Apache) are monitored and configurable

Early WIP. Commits have been squashed after large cleanups, hence short history.