`urlid` properties for fields

Suggestion here is to add a urlid property to field, which fills in for the predicate part of the triple

The Serializer will then need to be able to interpret incoming data based on their compact or absolute IRIs or their "default serialization" (where https://some-ontology/owl/#title is stripped to title)

It's probably better if the serializer only considers the expanded IRIs of properties, so that they're a single source of truth for a field/property

Note that sometimes multiple properties are equivalent (e.g. foaf:name and ns:fn)

(Changes every field on every model so this is no minor detail)

This is important for serializing the response to Turtle (#349 )

It's the issue here: documentation/specifications#5

It's the cause of all bugs like this one: djangoldp-account#69 (closed)