API does not allow filtering within embeddings

Created by: hermannsblum

The eve documentation claims to be able to do queries like GET /groupmemberships?where={"group.name"="admin"}. However, amivapi only returns an empty list.

Steps to reproduce curl -g 'https://amiv-api.ethz.ch/groupmemberships?where={%22$and%22:[{%22group.name%22:%22admin%22},{%22user%22:%2259e5f715edcb11000ae5ecd4%22}]}&embedded={%22group%22:1}' -H 'Authorization: 4XxU0WnjyVEiIZgMlwr9egSLmOXvnCMfJz-c2iO1ACk'