Re: [web-annotation] Rename `role` to `motive`

I'd vote that we stick with `role` in the JSON-LD and `oa:hasRole` in the
ontology.

If you were to use RDFa right now (and you can) to encode an annotation in
HTML you'd have use `oa:hasRole` (or its full URL) so there'd be no
confusion anyhow.

A "pure" HTML variant is also likely to only provide a subset of
functionally which is mappable to this ontology (vs. a lossless expression)
such that you'd likely never use `role` in the markup directly, but have
expressions that could be mapped properly as needed.

That's what I'd expect anyhow.