Fix #630: relative tokenUrl (#636)

* Fix #630: use relative tokenUrl as per the official recommendations

* Improve following review comments

* Fix unmatching backtick

* Improve consistency of authentication backend documentation

fastapi_users/authentication/jwt.py

@@ -30,7 +30,7 @@ class JWTAuthentication(BaseAuthentication[str]):
         self,
         secret: str,
         lifetime_seconds: int,
-        tokenUrl: str = "/login",
+        tokenUrl: str = "auth/jwt/login",
         name: str = "jwt",
     ):
         super().__init__(name, logout=False)