cubicweb-datacat #10771915 have RDF export compatible with DCAT-AP 1.1 [validation pending]

Once the underlying data model gets compatible with the recommended DCAT-AP 1.1 level, adjust the RDF export accordingly so that everything gets exported (and validated).

done in0.6.0
load left0.000
closed by<not specified>
patchAdd missing bits in test to have valid DCAT-AP 1.1 export [applied]Make a license document shareable among entities [applied]Add and export `dcat:theme` and `dcat:themeTaxonomy` [applied]Make some relations mandatory [applied]Add and export `dct:language` for catalogs [applied]Add and export `dct:format` and `dcat:mediaType` relations [applied]Export `dct:accrualPeriodicity` object as `dct:Frequency` [applied]