Bug 7813: Ability to delete local cover images

Bug 7813: Ability to delete local cover images

This patch adds the ability to delete local cover images from the
detail page. It adds an 'X' button to trigger the deletion. It occurs
using a new SVC script called cover_images that accepts the following
parameters:

 - action
 - biblionumber (used for checking imagenumber validity)
 - imagenumber (repeatable)

It then deletes all valid images passed and returns a JSON object
containing the imagenumbers and the deletion status (1/0). The operation
is triggered using jQuery.ajax, and with the response the corresponding
images get deleted.

Note: currently one image is deleted at a time, but the code is there to
be used.

To test:
- Open the detail page fr a biblionumber containing one or more cover
  images, go to its images tab.
- Apply the patch, reload the page, go to the images tab.
- Thumbnails should look better than before ;-)
- An 'X' button should show below each cover.
- Clicking the 'X' button should trigger an alert message asking for
  confirmation.
 a) Cancel: nothing happens, reload the page and check nothing got
    deleted.
 b) Accept: the deleted image should be gone, reload the page to check
    it doesn't exist anymore and is not a browser/DOM manipulation trick.
- Signoff

Note: check having more cover images than those that fit the width and
see it wraps fine.

P.S. I chose not to implement every possible action on the svc script, but
another bug could be filled for more improvements.

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>