cubicweb-skos #15922107 rdfio: proper canonical url [validation pending]

we should not use 'entity.absolute_url()' as default canonical url, as they may include changing data such as entity's name or title.

priorityimportant
typebug
done in1.0.0
load left0.000
closed by#184a488ff5e9 [rdf] Stop using entity.absolute_url() as canonical URI for an entity
patch[rdf] Stop using entity.absolute_url() as canonical URI for an entity [applied]