Fix refresh token key error for login

MB doesn't return refresh_token after first auth so use .get instead
to avoid KeyError.

listenbrainz/domain/brainz_service.py

@@ -25,7 +25,7 @@ def add_new_user(self, user_id: int, token: dict) -> bool:
             user_id=user_id,
             service=self.service,
             access_token=token["access_token"],
-            refresh_token=token["refresh_token"],
+            refresh_token=token.get("refresh_token"),
             token_expires_ts=expires_at,
             record_listens=False,
             scopes=self.scopes