misc correctness fixes

This PR tidies up some gnarly edges to the API to make the experience a little more polished.

54172203 exposes the fact that "crsid" is but one scheme understood by lookup. (Albeit, the only one understood in practice.) This doesn't change any behaviour but parameterises the previously hard-corded /people/crsid/{crsid} endpoint to people/{scheme}/{identifier}. This has little practical effect now but opens the possibility of allowing test users in future.

67a0b415 tidies up the Swagger documentation for the /people endpoint by making it appear as "people_list" in the documentation and correctly using a serialiser to generate the response.

6b3aebaf does the same with the institution list endpoint.