Convert activity_date_time field to datepicker and add support for url input

Convert activity_date_time field to datepicker and add support for url input

This converts the form to being a metadata defined field & a date-picker field as well
as adding standardised url support for input.

The input format is
activity_date_time_high=20100101

I stripped out the complex and probably broken based on feedback  & not used in core
existing hard-coded url support.

Note that there are some outstanding items
1) the upgrade script item needs to be added - this includes dealing with renaming of the field
since I made it more consistent
2) I stripped out the check to ensure the high date is greater than the low date
- if we still want this we should re-add as a generic thing. My guess is it's a bit optional