Bug 27703: Make ID of category select field unique

Bug 27703: Make ID of category select field unique

Duplicate element IDs on the authorized values page are invalid and
cause a problem with the JavaScript intended to submit the form when the
category select field changes.

This patch changes the ID of the label and the field and makes the
corresponding change to the JavaScript.

To test, apply the patch and go to Administration -> Authorized values.

 - Click an authorized value category to view its contents.
 - Using mouse or keyboard, make a selection from the "Show category"
   dropdown menu.
 - The form should submit automatically and direct you to the
   corresponding category.
 - Validate the HTML of the page and confirm that there are no errors.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>