Bug 30976: Display biblio's cover images first

Bug 30976: Display biblio's cover images first

If one add cover images at item's level, then biblio level, the cover images
of the items are displayed first on the main "cover slider" at the top of the page.
We should displayed biblio cover images first, then the ones for the items.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 6c55159eb6a91891f7c1becd49ec927ee9a3a36a)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 9938f61403dc6cd5abfd906eaf013bf7b53d9e5b)
Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>