Introducing main.sass stylesheet

Blender Cloud used main.css from Pillar

As we try to strip Blender Cloud-specific content from Pillar,
this commit brings three .sass files over to this repository.

There are still plenty of Blender Cloud classes all over Pillar,
they will be brought here over time.

src/templates/layout.pug

@@ -55,7 +55,7 @@ html(lang="en")
 		| {% if title == 'blog' %}
 		link(href="{{ url_for('static_pillar', filename='assets/css/blog.css', v=17320171) }}", rel="stylesheet")
 		| {% else %}
-		link(href="{{ url_for('static_pillar', filename='assets/css/main.css', v=17320171) }}", rel="stylesheet")
+		link(href="{{ url_for('static', filename='cloud/assets/css/main.css', v=17320171) }}", rel="stylesheet")
 		| {% endif %}
 		| {% endblock css %}