fixing thumbnail in meta tags, closes #290

templates/lecture.html

@@ -13,7 +13,7 @@
 {% block header %}
 {{ super() }}
 
-	<meta property="og:image" content="{{ config.VIDEOPREFIX }}/{{ lecture['titlefile'] }}" />
+	<meta property="og:image" content="{{ config.VIDEOPREFIX }}/thumbnail/l_{{lecture.id}}.jpg" />
 	<meta property="og:title" content="{{ course.title }}: {{ lecture.title}}" />
 	<meta property="og:description" content="{{ lecture.comment }}" />
 {% endblock %}