Bug 17524: Fix datepicker check for iso dates

Bug 17524: Fix datepicker check for iso dates

Iso dates are yyyy-mm-dd, the is_valid JS function
expects \d{2}-\d{2}-\d{4} when it should be \d{4}-\d{2}-\d{2}

Test plan:
Switch DateFormat to iso
Set a due date using the datepicker
=> Should work as expected

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 23f4830095d64d9a421a458fe5582930aa229aa7)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>