Direct link to external controlled vocabularies

[AFoletti]

DCAT has a list of controlled vocabularies we should use.

Today (see #23 and linked PRs) we integrate some of them directly in the schema.
However, some vocs are quite long and is impractical to integrate them fully in our schemas.
A better approach would be to let the catalog application know where to find those vocs, be able to retrieve the available values and finally present them to the user as drop-down.

Possible issues with this approach:

• Some vocs contain entries that are totally irrelevant for us. See for example the implementation of accrualPeriodicity, which is only a subset of what is available. How can we limit the choice to what is interesting for us?