Django-dcat

What's Changed
This realease adds a POC version of a jsonld serialization for models.

Calling `catalog.to_jsonld()` should generate a ready to publish dictionary with the most important mandatory fields of `Catalog`, `Agent`, `Dataset` and `Distribution`.

More fields will be added to the serializer in the future


**Full Changelog**: https://github.com/pdelboca/django-dcat/compare/v0.0.8...v0.0.9

- Add missing migration file that add help_text to all models.

- Add help_text to the models.
- Make property download_url optional.

- Catalog's Licence field should not be mandatory.

- File is no longer mandatory in Distributions (it could be an external URL)
- Add external_access_url to handle references to distribution hosted in other catalogs

Adds optional landing_page property to dataset