Improve the rendering of the control characters greatly by using

the Unicode counterparts in th 0x24## plane and add a little bit
of explanation what we are doing there and why we are doing this.

www/media/idf/css/style.css

@@ -663,8 +663,6 @@ table.diff-contents td > span.non-printable {
   color: white;
   text-transform: uppercase;
   float: none;
-  font-size: 5.5pt;
-  font-family: Calibri, Helvetica, Arial, sans-serif;
   text-align: center;
   display: inline-block;
   padding: 1px 1px 0px 1px;