Bug 28961: Use Flatpickr on tools pages

Bug 28961: Use Flatpickr on tools pages

This patch replaces the use of jQueryUI's datepicker on tools pages.

To test, apply the patch and test the following tools pages to confirm
that datepickers work correctly. "Linked" date fields should prevent a
"to" selection which preceeds the selected "from" date.

 - News -> New entry: Linked fields "Published date" and "Expiration
   date".
 - Batch extend due dates: Linked fields "Due date from" and  "Due date
   to"; "Hard due date" field.
 - Batch patron deletion: "who have not borrowed since," "whose
   expiration date is before," and "Permanently delete checkout history
   older than."
 - Export: Linked "Start date" and "End date" fields.
 - Import patrons -> Enter default values: "Date of birth,"
   "Registration date," and "Expiry date."
 - Inventory: "Set inventory date to."
 - Batch patron modification -> Submit a batch of cardnumbers or a
   patron list: "Registration date," "Expiry date," and "Restriction
   expiration."
 - Task scheduler: "Date" field.
 - Log viewer: "Display from" and "Display to" linked fields.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>