Bug 11793 - Use validation plugin when creating new item type

Bug 11793 - Use validation plugin when creating new item type

The page for adding a new item type 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 -> Item types -> New
item type. Try submitting the form with the following error conditions:

- Missing item type
- Missing description
- A non-number in the "rental charge" field

These errors should trigger a validation warning.

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

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>