Bug 15938 - Use validation plugin when adding or editing classification sources and...

Bug 15938 - Use validation plugin when adding or editing classification sources and filing rules

In Administration -> Classification sources there is some embedded
JavaScript which checks for empty form values. This patch removes that
JS and adds validation using the globally-included validation plugin
instead.

To test, apply the patch and go to Administration -> Classification
sources.

- Click to add a new classification source and confirm that you cannot
  submit the form with empty required values.
- Edit an existing classification source and confirm that you cannot
  empty a required field and submit the form.
- Repeat the previous two steps with filing rules.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>