MOTM-957: Only returns parent articles which are dossiers

Could return multiple parent articles if the articles was linked as a related article.
In this case only one result was returned and no checking was done.

Conflicts:
	newscoop/classes/ContextBoxArticle.php
Merge pull request #503 from m038/MOTM-957-stable

MOTM-957: Only returns parent articles which are dossiers