Offer a DCat representation for packages in the API

[amercader]

/api/rest/package/foo.rdf should return a DCat representation of the package. To create it, we will use the functions in ckanext-rdf.

[pudo]

I propose we do a bit of frameworkiness here: set up a two-part content type registry and refactor the API controller to use it.

The idea would be to do the following things:

1. Create a registry mapping format extensions to mime types: RepresentationRegistry?.add_format('json', 'application/json')
2. Create a registry of (mime types, entity types) -> converter functions that yield an appropriate representation of the entity.
3. Hook this into the _finish method of the API controller based on Accept: handling
4. Add support for /api/rest/ENTITY/NAME.{format} and /ENTITY/NAME.{format} to routing, use registry from (1) to rewrite accept headers.
5. Register converters in load_environment or via IConfigurable plugins
6. Document

What do you think?

[pudo]

Now available in ckanext-rdf and linked to from WUI.