style: use monospace for 'formatted-fixed' class

The 'formatted-fixed' class is applied to PR comments, PR descriptions and
repository descriptions. It already preserves whitespace via 'whitespace:
pre-wrap', but the font is not monospace. As a result, diagrams or other
ASCII art do not display well.

Use monospace on this class to fix that.

kallithea/public/less/style.less

@@ -9,6 +9,7 b' body {'
 
 /* class for texts where newlines should be preserved, for very light-weight ascii art markup (like pull request descriptions) */
 .formatted-fixed {
+  font-family: @font-family-monospace;
   white-space: pre-wrap;
 }