Fix styling for line numbers

Need to use .CodeMirror to override specificity of CM styles. This
should mean that CM styles use serifed font not monospaced

services/web/public/stylesheets/app/editor/rich-text.less

@@ -1,11 +1,11 @@
 @rt-font-family: 'Source Sans Pro', 'Helvetica', 'Arial', sans-serif;
 // @rt-font-family-serif: 'Palatino Linotype', 'Book Antiqua', Palatino, serif;
 
-.rich-text {
+.rich-text .CodeMirror {
   font-family: @rt-font-family;
   font-size: 1.15em;
 
-  pre, .CodeMirror-linewidget {
+  pre {
     font-family: @rt-font-family;
   }