Bug 28180: (follow-up) Updates to styling of lightbox controls

Bug 28180: (follow-up) Updates to styling of lightbox controls

This patch adds style changes like the ones made in Bug 28179. It also
addresses a bug with the way the verify_images() function was firing.
I've moved the function outside of document.ready() and added a call to
it on window load, consistent with how it works in the staff client.

I've removed a redundant call to verify_images() from the footer include
file, opac-bottom.inc.

The cosmetic changes replace the Chocolat image icons with new SVG
assets based on Bootstrap Icons (https://icons.getbootstrap.com/). The
color changes I think help the controls to be more visible.

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