[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.
