Reject more invalid usernames from clients.

Specifically this should cover people connecting with a username of
"\u200C"

common/pb/event_connection_closed.proto

@@ -9,6 +9,7 @@ message Event_ConnectionClosed {
 		SERVER_SHUTDOWN = 2;
 		TOO_MANY_CONNECTIONS = 3;
 		BANNED = 4;
+		USERNAMEINVALID = 5;
 	}
 	optional CloseReason reason = 1;
 	optional string reason_str = 2;

common/pb/response.proto

@@ -22,6 +22,7 @@ message Response {
 		RespChatFlood = 18;
 		RespUserIsBanned = 19;
 		RespAccessDenied = 20;
+		RespUsernameInvalid = 21;
 	}
 	enum ResponseType {
 		JOIN_ROOM = 1000;