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.

CHANGELOG.md

@@ -1,10 +1,13 @@
 CHANGELOG
 =========
 
+This branch supports Ubuntu 18.04 **only**. When upgrading, **always** upgrade your **existing** Ubuntu 14.04 machine to version the latest release supporting Ubuntu 14.04 --- v0.28. If you are running an older version of Mail-in-a-Box which has an old version of ownCloud or Nextcloud, you will *not* be able to upgrade your data because older versions of ownCloud and Nextcloud that are required to perform the upgrade *cannot* be run on Ubuntu 18.04.
+
 In Development
 --------------
 
 * Starting with v0.28, TLS certificate provisioning wouldn't work on new boxes until the mailinabox setup command was run a second time because of a problem with the non-interactive setup.
+
 * Update to Nextcloud 13.0.5.
 * Update to Roundcube 1.3.7.
 * Update to Z-Push 2.4.4.