Add `PUT` method support for `/referrers` endpoint

[dmcgowan]

The fallback approach adds many advantages for clients and servers. Having a dedicated /referrers endpoint adds some advantages over the current fallback approach. We can define referrer specific headers, suggest specific validation, and avoid any extra requests from clients when getting referrers. Adding PUT would allow maintaining the advantages of fallback approach with the advantages of having referrers served from a single endpoint.