[web/facet] Use an Exists node in HasRelationFacet The generated RQL now includes an EXISTS clause. For instance, one gets: :: DISTINCT Any WHERE X is CWUser, EXISTS(X in_group A) instead of: :: DISTINCT Any WHERE X is CWUser, X in_group A for a HasRelationFacet with ``rtype = 'in_group'``, which is more appropriate for testing the existence of the relation. Add a test for this facet along the way.

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