Bug 28958: Use Flatpickr on serials pages

Bug 28958: Use Flatpickr on serials pages

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

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

- Claims -> Choose vendor -> Filter missing issues (linked "from" and
  "to" fields).
- Check expiration -> "Expiring before" field.
- Manage numbering patterns -> New numbering pattern -> "First issue
  publication date" field.
- Serials advanced search -> "Expires before" field.
- Subscription search results -> Check multiple checkboxes -> Edit
  selected serials: "Expiration date" field.
- Subscription details -> Serial collection -> Edit serials ->
  "Published on" and "Expected on" fields.
- Subscription details -> Renew -> "Start date" field in the popup
  window.
- Subscription details -> Edit subscription -> Page 2 -> "First issue
  publication date," "Subscription start date," and "Subscription end
  date" fields.
- Locate a subscription with manual history enabled. From the
  subscription detail page, click the "Planning" tab, then "Edit
  history" -> Linked fields "Subscription start date" and "Subscription
  end date."

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