Media Entity type no longer extends hidden fields

The Media Entity type no longer extends the hidden field to enable better confirguration of the label and possition
Sonata form mapper callback now used

Now posssible to use the call back method to build form fields for a content type, will be called after field definitions from annotations are loaded, which is the same behaviour as in the iBrows bundle
datetimepicker js moved to field

This allows for loading when form is loaded via ajax