Improvements on xhtml and css generation

* Load the content in a way which handles xhtml and css better
* Get rid of the static dictionary and move the class override
  into the model so that minor entries can use a different class
  name than normal entries
* Add a refresh of the preview when the model is saved

Change-Id: Id7f1b79bcbb8aa78c40b3fafe73c826df96dfd0c