RDF view and export

[KonradHoeffner]

As discussed with @ChristophB, viewing and exporting resources as RDF would be nice.

• [x] decide, which types of resources to map
• [x] find suitable vocabularies and properties
• [x] create mapping
• [x] decide on a namespace
• [ ] wait for https://github.com/Onto-Med/top-api/issues/178
• [ ] allow user to view, copy and download serialized RDF, for example as RDF Turtle
• [ ] optionally provide an HTML view similar to RickView (however RickView itself isn't suitable for dynamically generated resources)

[ChristophB]

I have moved the first four tasks into another issue of the TOP API. (see https://github.com/Onto-Med/top-api/issues/178)