minimal changeset to get things working on 18.04

@joshdata squashed pull request #1398, removed some comments, and added these notes:

* The old init.d script for the management daemon is replaced with a systemd service.
* A systemd service configuration is added to configure permissions for munin on startup.
* nginx SSL settings are updated because nginx's options and defaults have changed, and we now enable http2.
* Automatic SSHFP record generation is updated to know that 22 is the default SSH daemon port, since it is no longer explicit in sshd_config.
* The dovecot-lucene package is dropped because the Mail-in-a-Box PPA where we built the package has not been updated for Ubuntu 18.04.
* The stock postgrey package is installed instead of the one from our PPA (which we no longer support), which loses the automatic whitelisting of DNSWL.org-whitelisted senders.
* Drop memcached and the status check for memcached, which we used to use with ownCloud long ago but are no longer installing.
* Other minor changes.

setup/management.sh

@@ -87,16 +87,16 @@ rm -f /tmp/bootstrap.zip
 
 # Create an init script to start the management daemon and keep it
 # running after a reboot.
-rm -f /usr/local/bin/mailinabox-daemon # old path
+rm -f /usr/local/bin/mailinabox-daemon /etc/init.d/mailinabox # old paths
 cat > $inst_dir/start <<EOF;
 #!/bin/bash
 source $venv/bin/activate
 exec python `pwd`/management/daemon.py
 EOF
 chmod +x $inst_dir/start
-rm -f /etc/init.d/mailinabox
-ln -s $(pwd)/conf/management-initscript /etc/init.d/mailinabox
-hide_output update-rc.d mailinabox defaults
+hide_output systemctl link conf/mailinabox.service
+hide_output systemctl daemon-reload
+hide_output systemctl enable mailinabox.service
 
 # Remove old files we no longer use.
 rm -f /etc/cron.daily/mailinabox-backup