Bug 27135: Adjust "upload image" button when viewing item's cover images

Bug 27135: Adjust "upload image" button when viewing item's cover images

When viewing the cover image of an item, the "upload image" button is
about to upload an image at biblio level.

Test plan:
- Attach a cover image to an item
- View it
- Notice the "Upload image" button, confirm that with this patch applied
the wording changed and that clicking it will attach a cover image to
the item
- Attach a cover image to the biblio
- View it
- Notice the "Upload image" button, confirm that with this patch applied
the wording changed and that clicking it will attach a cover image to
the biblio

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>