Add markdown utils

- External links open in a new tab
- Clicking on image open an image modal

templates/problem/submission-diff.html

@@ -12,7 +12,6 @@
       });
     });
   </script>
-  <script type="text/javascript" src="{{ static('libs/featherlight/featherlight.min.js') }}"></script>
   <script type="text/javascript">
   </script>
 {% endblock %}