Bug 11787 - Use validation plugin when adding new vendor

The vendor entry form uses some custom JavaScript which can be removed
in favor of HTML5 validation attributes and Koha's built-in validation
plugin. This patch does so.

Also corrected: Minor validation issue.

To test, apply the patch and go to Acquisitions -> New vendor. Try
submitting the form without entering a vendor name. This should trigger
a validation warning.

Submission of the form with valid data should work correctly. Editing an
existing vendor 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>
(cherry picked from commit 03675fd55320f3a9bc3d4c7475bc3b9696920bd9)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>