Bug 25031: (QA follow-up) Improve handling of one or fewer images

Bug 25031: (QA follow-up) Improve handling of one or fewer images

This patch addresses a few issues raised during QA:

1. If there are no images, avoid the momentary appearance of an empty
   borered box. The class controlling the appearance of the box is now
   added after initialization.
2. If there is only one image, remove the control for switching
   between covers.
3. Add "preventDefault" to cover naviation click handler.
4. Correct translation function in localcovers.js.

This patch modifies SCSS, so rebuilding the staff client CSS is
necessary for testing.

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