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
Fixes pages_controller etag and last-modified headers.

The current implementation always delivers the same etag, regardless of the logged in state of a user.
If a user is logged in and has sufficient rights to edit a page (aka. an author), the menu bar will be displayed while browsing the site. The problem is, that this will be cached by http caches like varnish and rack/cache.

This fix changes the etag so it reflects the current users state.

Also this fix ensures that a last-modified header gets returned, even if published_at was never set, aka, it's nil. In this case it will return updated_at.
Only try to get current_alchemy_user cache_key for page etag. Refs #498

We do not know, if the user class always is an instance of ActiveRecord, so we cannot be 100% sure to have the `cache_key` method.
Fixes messages in dialogs without padding.
Adding name to "missing-content" selector. Fixes #499