Bug 30514: Error in date format check following datepicker removal

Bug 30514: Error in date format check following datepicker removal

This patch corrects the is_valid_date function in the OPAC and staff
interface so that it work correctly with Flatpickr.

To test, apply the patch and clear your browser cache if necessary.

- In the staff client, locate a bibliographic record and edit its items.
- Test that the "Date acquired" date picker works correctly and that
  there are no errors in the browser console.
- Test other date input fields to check that they still work, e.g.
  specify due date during checkout; Catalog statistics wizard; Patron
  entry/modification, etc.
- Perform similar tests in the OPAC: Hold suspension; Update your
  personal details.

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>