[IMP] website_(profile, slides): use badge level image as default badge image

Before this commit, if no image was set on the badge, no image was shown when
displaying the badge.

After this commit, if a badge level is set on the badge and no image is set,
the badge level image will be used for the badge.

This fix / improvement is applied twice

  * when displaying an user profile, its received badges;
  * when displaying current user achievements in eLearning;

Task ID 1981399
PR #43577