Adds a "missing contents"-link observer, that sends the request via real ajax post.

Unfortunately Rails does not send a remote ajax request with a method set to 'post' as real post request.

This function converts the query string into a real post body. This fixes issues with missing content links having a lots of option values. I.e. Lots of select values in a EssenceSelect.
Convert cursor to pointer if hovering an element in preview frame.
Fixes several issues with EssenceDate editor.

* The preview_text was missing an argument
* The jQuery-UI Datepicker select boxes where broken in Firefox.
* Passes `default_value` option to `alchemy_datepicker` helper