Make <meta property="og:url"> tags have an absolute URL

Most of those can simply use `{{ request.url }}`, as this already contains
the absolute URL of the currently displayed page.

src/templates/projects/landing.pug

@@ -51,7 +51,7 @@ meta(name="twitter:description", content="Blender Cloud, your source for open co
 
 | {% endif %}
 
-meta(property="og:url", content="{{url_for('projects.view_node', project_url=project.url, node_id=node._id)}}")
+meta(property="og:url", content="{{url_for('projects.view_node', project_url=project.url, node_id=node._id), _external=True}}")
 | {% else %}
 meta(property="og:title", content="{{ project.name }} Blog on Blender Cloud")
 meta(name="twitter:title", content="{{ project.name }} Blog on Blender Cloud")