Switch to serif font for rich text

Makes rich text ressemble a manuscript more closely

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

@@ -1,12 +1,11 @@
-@rt-font-family: 'Source Sans Pro', 'Helvetica', 'Arial', sans-serif;
-// @rt-font-family-serif: 'Palatino Linotype', 'Book Antiqua', Palatino, serif;
+@rt-font-family-serif: 'Palatino Linotype', 'Book Antiqua', Palatino, serif;
 
 .rich-text .CodeMirror {
-  font-family: @rt-font-family;
+  font-family: @rt-font-family-serif;
   font-size: 1.15em;
 
   pre {
-    font-family: @rt-font-family;
+    font-family: @rt-font-family-serif;
   }
 
   // TODO: Change prefix away from wl- ?