Bug 11791 - Use validation plugin when creating new city

Bug 11791 - Use validation plugin when creating new city

The page for adding a new city includes some custom form
validation JavaScript which can be removed in favor of HTML5 validation
attributes and Koha's built-in validation plugin. This patch does so.

To test, apply the patch and go to Administration -> Cities -> New city.
Try submitting the form without entering a city or zip code. This should
trigger a validation warning.

Submission of the form with valid data should work correctly. Editing an
existing city should also work correctly.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Note: This patch changes the behavior. Before the patch, only 1 out of city
and zip was required. Now both are. Since the 2 inputs were marked as
required, I think they should be.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>