Bug 29228: Use Flatpickr on offline circulation page

Bug 29228: Use Flatpickr on offline circulation page

This patch replaces the use of jQueryUI datepicker on the built-in
offline circulation page, where it was used for the "Specify due date"
date-time picker as well as for formatting datetime values for display.

To test, apply the patch and go to Circulation -> Built-in offline
circulation interface.

 - Click "Synchronize."
 - Click "Download records."
 - Submit a card number for a patron who has multiple items checked out.
 - In the table of previous checkouts, confirm that dates in the "Date
   due" and "Checked out on" columns are formatted according to your
   TimeFormat and DateFormat system preferences.
 - When checking out to a patron with fines, open the fines tab and
   submit an amount in payment. The table of payments should include a
   correctly-formatted datetime.
 - Test that the "Specify due date" date picker works correctly and that
   checkouts show the correct date.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>