related: pcsg/buero#438

feat(ArticleView): add new method getImageUrl

See merge request !128

(cherry picked from commit 275e8d6c)

Co-authored-by: Michael Danielczok <michael@pcsg.de>

Related: brassberg/tickets#244


(cherry picked from commit b0ddd625)

Co-authored-by: Michael Danielczok <michael@pcsg.de>

This trait class is temporary, we may switch to interfaces later.
this trait class extends erp entities with getReferenceData().
this must be used wherever an erp is to be stored in extra data.

related: pcsg/buero#416