Bug 11792 - Use validation plugin when creating new currency

Bug 11792 - Use validation plugin when creating new currency

The page for adding a new currency 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 -> Currencies &
Exchange rates -> New currency. Try submitting the form without entering
a currency, rate, and/or symbol. This should trigger a validation warning.

Submission of the form with valid data should work correctly. Editing an
existing currency 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>