Validate Harder

CAUTION: This change will make importing data harder, and if applied after data is imported will make editing existing datasets difficult due to resource validation failing.

Please merge with care.

* organizations are now actually required
* resource fields are now actually required
* resource format has new "Other" option (French translation needed)
* choice fields validate that the value sent matches one of the choices
* some fields are now required depending on the catalog_type
* mandatory field in schema description is now documented

The choice field validation can't be overridden by superusers, but the other required fields can, at least through the API.