Bug 28179: (follow-up) Updates to styling of lightbox controls

Bug 28179: (follow-up) Updates to styling of lightbox controls

This patch fixes a bug and adds some style changes which I think are
nice. The bugfix is to change the z-index of the lightbox footer so that
it isn't hidden when there is a language menu in the footer.

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.

License information about Bootstrap Icons has been added to the About
page.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
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>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>