FB open graph does not support SVG images only use png/jpg/jpeg on gu…

…ide pages

- Add explicit width/height and description to satisfy open graph debugger warnings
- This will fallback to the user's avatar image if the stack image isn't the
  right format.
- We're planning on transitioning to png for stack images so this should
  automatically work once that change happens.
    - Follow #89 for more discussion on that topic.

pskb_website/templates/article.html

@@ -7,9 +7,12 @@
     <meta property="og:type" content="article" />
     <meta property="og:title" content="{{article.title}}" />
     <meta property="og:url" content="{{canonical_url}}" />
+    <meta property="og:description" content="Guide related to {{article.stacks[0]}}" />
 
-    {% if article.stack_image_url %}
+    {% if article.stack_image_url and article.stack_image_url.endswith(('.png', '.jpg', '.jpeg')) %}
         <meta property="og:image" content="{{article.stack_image_url}}" />
+        <meta property="og:image:width" content="400" />
+        <meta property="og:image:height" content="400" />
     {% endif %}
 
 {% endblock %}