Bug 11785 - Use validation plugin when uploading local cover images

Bug 11785 - Use validation plugin when uploading local cover images

When uploading local cover images the form should not be submitted if
no file has been selected. The existing form validation script doesn't
work. This patch adds HTML5 validation attributes and use of Koha's
built-in form validation plugin.

To test, apply the patch and go to Tools -> Upload local cover image.
Try to submit the form without selecting a file to upload. You should be
prevented from doing so. Choose a file and confirm that the upload
completes 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 7faff33c8b9bd670cf7813f4cefb7aee38709ebb)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>