coal: use preformatted text for descriptions (issue2835)

The coal style was broken here by an earlier change to paper, which
shares files.

mercurial/templates/static/style-coal.css

@@ -221,6 +221,8 div.description {
   border-left: 3px solid #999;
   margin: 1em 0 1em 0;
   padding: .3em;
+  white-space: pre;
+  font-family: monospace;
 }
 
 /* Graph */