Merge branch 'fix-anchor-icon' into 'master'

Fix anchor icon regression

An error in scss prevented the markdown header `:hover` anchor icon svg from linking correctly:

![Screen_Shot_2016-09-14_at_4.06.19_PM](/uploads/f4d31a17a58bd7cbde572f91c316dfe1/Screen_Shot_2016-09-14_at_4.06.19_PM.png)

The following currently 404s:  
<https://gitlab.com/assets/icon_anchor.svg>  

## What are the relevant issue numbers?

#20459 & !5577

See merge request !6358

app/assets/stylesheets/framework/typography.scss

@@ -164,7 +164,7 @@
       text-decoration: none;
 
       &:after {
-        content: url('icon_anchor.svg');
+        content: image-url('icon_anchor.svg');
         visibility: hidden;
       }
     }