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.

conf/nginx.conf

@@ -31,8 +31,8 @@ server {
 
 # The secure HTTPS server.
 server {
-	listen 443 ssl;
-	listen [::]:443 ssl;
+	listen 443 ssl http2;
+	listen [::]:443 ssl http2;
 
 	server_name $HOSTNAME;