Make article:author OpenGraph compatible

OpenGraph actually needs a profile URL as value to article:author.
We point it to the author's page and adjust the OG meta tags on
author.html to include the relevant profile information.

templates/author.html

@@ -2,6 +2,15 @@
 
 {% block title %}{{ SITENAME }} - Articles by {{ author }}{% endblock %}
 
+{% block opengraph %}
+    {{ super() }}
+    <meta property="og:title" content="{{ SITENAME }} - Articles by {{ author }}">
+    <meta property="og:type" content="profile">
+    <meta property="profile:first_name" content="{{ author.name.split(' ')[0] }}">
+    <meta property="profile:last_name" content="{{ author.name.split(' ')[1:]|join(' ') }}">
+    <meta property="profile:username" content="{{ author.slug }}">
+{% endblock opengraph %}
+
 {% block header %}
     <!-- Page Header -->
     {% if HEADER_COVER %}