HTTPS still loading insecure content

My goal is simple, everything should be served over https. Just like this site.

Aparently the path to that goal isn't as simple as I thought.

I set my apache vhost to rewrite everything to https.

  RewriteEngine On
  RewriteCond %{HTTPS} off
  RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}

In the config.php file I set this:

$CFG->wwwroot   = 'https://ws-test-moodle-01.example.com';
$CFG->httpswwwroot = 'https://ws-test-moodle-01.example.com';

That part works fine. But it's still loading some insecure content. Thus, instead of a green padlock, I get a grey globe, in Firefox, and a grey padlock in Chrome.

So, how do I load all content securely?

I'm on Moodle 2.2.5+, Ubuntu 12.04.