This requires changes to the database secrets, which now have to have
an item with a key that matches the value of a line in each
`sql*.conf.ext` file formatted as:

> # dbname: <KEY>

Each item must additionally contain a "host" value as well as the
previously required "username" and "password" values.  Optionally
a "dbname" value could be supplied to override the database name in the
items's key.

This query is needed for full synchronisation between mailbox servers.

Closes: #31

Dovecot no longer has even any knowledge of SSLv2.

Fixes: #15

Allows index directories for subscribed mailboxes to be created
automatically.

Fixes #12

And get the other settings from a script embedded in the service YAML.