Remove unnecessary margins in social links

On small screens the list is on its own line so it doesn't need a
margin, while on medium/large screens the padding of its parent element
makes the margin unnecessary as well.

app/assets/stylesheets/layout/social.scss

@@ -6,6 +6,10 @@
     width: 1 * 100% / 3;
   }
 
+  ul {
+    margin: 0;
+  }
+
   li {
     display: inline-block;
   }