[facets] support for `no_relation` on RelationFacet
e.g. to filter en entities *without* a given relation as well as with a
particular relation. Proposed or not by default according to relation
cardinality.
Also, try to guess i18nable properly from the schema by default.
Upstream Author:
%(author)s <%(author-email)s>
Copyright:
Copyright (c) %(year)s %(author)s.
%(author-web-site)s -- mailto:%(author-email)s