dovecot: migrate to dedicated homedir and separate maildir paths

Per the dovecot documentation[0] we were previously running with an
unsupported home directory configuration, because we shared them among
all virtual users at /var/vmail.

After resolving this by creating per user home directories at
/var/vmail/%{domain}/%{user} this now also overlaps with the location of
the Maildir, which is not recommended.

As a result we now need to migrate our Maildirs into
/var/vmail/%{domain}/%{user}/mail, for which a small shell script is
provided as part of this change.

The script is included in the documentation because we cannot provide it
in time for users, because they might already be seeing the relevant
assertion and there is no safe waiting period that would allow us to skip
shipping it like that.

[0] https://doc.dovecot.org/2.3/configuration_manual/mail_location/

mail-server/assertions.nix

@@ -38,6 +38,16 @@
             '';
           }
         ]
+    ++ [
+      {
+        assertion = config.mailserver.stateVersion >= 3;
+        message = ''
+          Issue: The dovecot mail location for all users has changed and need to be migrated.
+
+          Check https://nixos-mailserver.readthedocs.io/en/latest/migrations.html#dovecot-mail-directory-migration for the required remediation steps.
+        '';
+      }
+    ]
     ++ lib.optionals (config.mailserver.certificateScheme != "acme") [
       {
         assertion = config.mailserver.acmeCertificateName == config.mailserver.fqdn;