style: use monospace on all multi-line form inputs

Multi-line form input (textarea) is used for PR comments, PR descriptions,
repository descriptions and 'HTML/CSS/Javascript customization block'.
In the first three cases, the rendered result is displayed in monospace, so
the corresponding textarea should be as well.
As the expected content of the HTML/CSS/Javascript block is code, it makes
perfect sense to use monospace there too.

kallithea/public/less/style.less

@@ -194,6 +194,7 b' table.changesets {'
 
 /* textareas should be at least 100px high and 400px wide */
 textarea.form-control {
+  font-family: @font-family-monospace;
   min-height: 100px;
   min-width: 400px;
 }