send_custom_email: Improve printing of outgoing emails.

The fake email addresses are not useful for debugging/confirmation,
and additionally it's nice to have the realms listed as well.

zerver/management/commands/send_custom_email.py

@@ -91,4 +91,4 @@ class Command(ZulipBaseCommand):
         if options["dry_run"]:
             print("Would send the above email to:")
             for user in users:
-                print(f"  {user.email}")
+                print(f"  {user.delivery_email} ({user.realm.string_id})")