Add "picture" to the /userinfo endpoint. (#1914)

* Add "picture" to the /userinfo endpoint.

This may be null or a URL.
 add picture for the test authentication method

server/auth/auth.py

@@ -43,6 +43,10 @@ class AuthTypeBase(ABC):
         """Return the name of the user (string)"""
         pass
 
+    def get_user_picture(self):
+        """Return the location to the user's picture"""
+        return None
+
 
 class AuthTypeClientBase(AuthTypeBase):
     """Base type for all authentication types that require the client to login"""