Add option to ignore links without thumbnails

[arnowelzel]

During the removal of jQuery as dependency, the frontend script was also changed to use all links with lbwsp-attributes and not only those which contain an img element as child.

There may be usecases where you don't want to have plain links to an image to open with a lightbox. It should be possible to choose wether all links should be used or of text links without visible image should be ignored.