Add a cache purge

Uses the `/remove/%Username%` endpoint.

Possibly overly simplistic in that it does not check for the username in the cache before deleting it (doesn't cause any visible errors though if it doesn't exist). Also will therefore report it has been removed when it may have not existed.

It responds with a `204 No Content` because it doesn't really need to respond with anything...

I have tested with both Redis and Memory, works.

Kind of was easier than I thought to implement :tractor: