Don't allow clients who are already authenticated to a session to join the session as a new user.

[RocketPuppy]

If the client has already authenticated as a Questioner or Answerer on a session, don't allow them to re-join the session as a new user. This effectively means checking for the presence of the authentication cookies when joining a session.

Allowing this behavior means clients must explicitly join a session as an unauthenticated user.

See #18 for the opposite functionality.

[RocketPuppy]

I looked into this, but I'm not sure it's really desired. At the minimum, it shouldn't be handled by the server.