Bug 14060: Remove readonly attributes on date inputs

Bug 14060: Remove readonly attributes on date inputs

In order to remove accessibility issues due to the readonly attributes
on date inputs, this patch will remove them and introduce a javascript
validation on them.

This patch is not perfect for some reason:
I didn't manage to force the user to select a valid date. One solution
would be to reopen the datepicker plugin until a valid date is inserted.
But it could be annoying for users (and for me: I did not manage to
implement this solution).
You will note that input is emptied if the date is not valid. This is a
quick and efficient solution to prevent submitting invalid date and make
Koha explodes. A proper solution would be to implement the check server
side send a friendly message to the user.

Test plan:
For all inputs, try an invalid and a valid date.
 1/ Debar a patron
 2/ On the checkout tables (circulation and moremember), add a renewal
due date (at the bottom of the tables)
 3/ On the checkout page, specify a due date
 4/ On the return page, specify a return date
 5/ On the invoice page (acquisition module), enter a shipment and
billing date
 6/ On the invoice search page (invoices.pl) use filters shipment and
billing dates
 7/ On the offline circ page, specify a due date
 8/ On the edit patron page (memberentry), add a debarment
 9/ On the reserve page (reserve/request.pl), use the date inputs to
suspend until a defined date
10/ Edit patrons in a batch (tools/modborrowers.pl) and use the
registration and expiry date inputs

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>