Bug 7813: (follow-up) improvements for deleting local cover images

Bug 7813: (follow-up) improvements for deleting local cover images

This patch:
- displays a js alert if a problem occurred on deleting the
  image on the server.
- adds a class for the new span (in order to avoid future problem, if
  someone adds a new span).
- adds a title for the [X] link.
- deal with the upload_local_cover_images permission (don't display the
  delete link if the logged in user does not have it).
- changes the license version.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
I like the ehancements to the original patch and work as expected.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>