Support the OpenGraph protocol

In order to properly support sharing of articles/blog posts in social media (e.g. Twitter) with proper metadata we should support the OpenGraph protocol: https://ogp.me/

Example:

<meta property="og:title" content="Title" />
<meta property="og:type" content="article" />
<meta property="og:url" content="https://software.hifis.net/test/test" />