Bug 26505: Suspend hold modal broken in the OPAC

Bug 26505: Suspend hold modal broken in the OPAC

This patch makes corrections to the markup for the suspend hold modal in
the OPAC. It hadn't been updated during the upgrade to Bootstrap 4.

The JavaScript controlling the "suspend until" datepicker has been
modified because the datepicker pop-up was not positioned correctly in
this new version.

Unrelated: The markup for confirmation modals has been updated to
replace Bootstrap 3's "btn-default" with Bootstrap 4's "btn-secondary."

To test, apply the patch and make sure the SuspendHoldsOpac and
AutoResumeSuspendedHolds system preferences are enabled.

- Log in to the OPAC as a user who has holds.
- On the "Your summary" page open the "Holds" tab.
- In the list of holds, click the "Suspend" button.
- The modal should appear and look correct.
- Test the "Suspend until" field: Clicking in the form field should
  trigger the datepicker. It should be positioned correctly under the
  form field.
- Confirm that the datepicker populates the field.
- Submit the suspension and confirm that the hold is suspended.
- Click the "Cancel" button for a hold. Confirm that the confirmation
  dialog appears correctly.

Edit: Updated class of hidden submit button. Test the page with JS
disabled to test that the "Suspend until" form works correctly.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>