a big refactoring:

 - remove unused column formatted_html from pages table
 - use a virtual attribute on the model instead, and remove the separate
   build_formatted_html hook method when saving
 - check explicitly if a previous page with the same title exists in
   PagesController#create, and if yes, pass that previously-existing page to the
   re-rendered :new view
 - provide a link to the previously-existing page in the "page already exists"
   error message for the :new view
 - refactor other parts of the form error display as well: messages, a separate helper method,
   removing flash div from layout and using a specific error display block in new/edit,  ...
 - add a couple of controller tests to specifically check the case of submitting an update
   with an empty title