Enable SSL for connections to database

Since the database can be exposed to the outer world, it seems better to
have an encrypted connection to it.
We automatically generate a self-signed certificate since it's only used
to encrypt the connection but one can easily replace it by a custom one,
just overwrite `server.crt` and `server.key` in the
/var/lib/dokku/services/postgres/DB_NAME/data directory.

scripts/enable_ssl.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+cd /var/lib/postgresql/data
+openssl req -new -newkey rsa:4096 -x509 -nodes -out server.crt -keyout server.key -batch
+chmod 600 server.key
+sed -i "s/^#ssl = off/ssl = on/" postgresql.conf
+sed -i "s/^#ssl_ciphers =.*/ssl_ciphers = 'AES256+EECDH:AES256+EDH'/" postgresql.conf