Before this diff, we completely ignored the `keyserverUsername`. The idea was that the user would enter another user's username there, and that the client would then query the identity service to ask how to talk to that keyserver.
Since the identity service isn't ready yet, instead we're going to have the user enter a URL prefix for the keyserver they want to talk to. This diff makes that change, and also calls the new version endpoint to make sure that the keyserver URL that the user entered points to a valid keyserver.
Depends on D8191