message_fetch: Use email as operand for 'dm' operator.

In our web client code we use email as the operand for
'dm' operator. Later user_ids_array is set as operand
in 'handle_operators_supporting_id_based_api' before
making the API call.

Earlier, using user_id as the operand was resulting in
the following error during user signup:
data=b'{"result":"error","msg":"Invalid narrow[0]: Value error,
operand is missing","code":"BAD_REQUEST"}

This commit replaces user_id with email as operand.

web/src/tutorial.js

@@ -18,7 +18,7 @@ export function initialize() {
             [
                 {
                     operator: "dm",
-                    operand: people.WELCOME_BOT.user_id,
+                    operand: people.WELCOME_BOT.email,
                 },
             ],
             {trigger: "sidebar"},