Add DCAT-AP HVD properties in RDF output if the dataset is tagged hvd

[maudetes]

• DCATAP.applicableLegislation (at the dataset and distribution level). See in particular this DCAT-AP HVD specification.
• DCATAP.hvdCategory (at the dataset level). See in particular this DCAT-AP HVD specification.

These properties should be added on dataservices when exposed as well.

We've added a support for tag filtering on the catalog, in order to do http://dev.local:7000/api/1/site/catalog.xml?tag=hvd. You can test using this output locally, and by having datasets tagged hvd and mobilite for example.

[!NOTE]
We rely heavily on EU_HVD_CATEGORIES french vocabulary as pivot for keywods. Should we move it to settings.py to be overriden in other udata instances?

Iterate on https://github.com/datagouv/data.gouv.fr/issues/1371.