Replace jcalendar instances with datepicker
We have been phasing out jcalendar since ? 4.x? there are a handful of instances left to switch over.
Once they are all gone we should be able to remove some php handling - datepicker handles date format at the js layer meaning the php/ form layer only needs to handle one format of dates
Note that removing jcalendar will remove a number of points of fragility as the datepicker is more robust