Bug 16112: (follow-up) Form style improvements, clear date

Bug 16112: (follow-up) Form style improvements, clear date

This follow-up makes some markup and style improvements in order to make
the renew form more consistent with similar forms like checkout and
check-in.

It also adds a "Clear" button for the datepicker field, matching similar
date inputs.

To test, apply the patch and go to Circulation -> Renew.

 - With or without the  SpecifyDueDate system preference enabled the
   form should look correct.
 - With the SpecifyDueDate preference enabled you should see the date
   form below the barcode input field.
 - Select a date and test that the "Clear" button works to clear it.
 - Compare to the check-in page to confirm that they look consistent
   with each other.

Signed-off-by: Marco Abi-Ramia <marco.abi-ramia@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>