Bug 28180: Bug fix - cannot select cover-nav button when on top of the .contentblock

Bug 28180: Bug fix - cannot select cover-nav button when on top of the .contentblock

Found a bug and ended up with this solution, suggestions welcomed!

example with OpenLibrary with isbn=9780670026623, the image is a bit
higher than others and then the cover-nav buttons are on top of
.contentblock and cannot be clicked

Sponsored-by: Gerhard Sondermann Dialog e.K. (presseplus.de, presseshop.at, presseshop.ch)
Signed-off-by: Rasmus Leißner <rasmus.leissner@solutions-factory.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>