Moves watermark link to be css variable.

css/_base.scss

     display: none;
     left: $defaultToolbarSize;
     margin-left: 10px;
-    background-image:url(../images/watermark.png);
+    background-image: url($defaultWatermarkLink);
     background-position: center left;
 }
 

css/_variables.scss

  * Misc.
  */
 $borderRadius: 4px;
+$defaultWatermarkLink: '../images/watermark.png';
 
 /**
  * Z-indexes. TODO: Replace this by a function.