Refactor code in `/tasks/admin_or_owner`

[ktirumalafb]

If you look here: https://github.com/facebookresearch/dynabench/blob/main/api/controllers/tasks.py#L241 the default is that we return True unless the if statements are satisfied, I think it should be the other way around (we start off with False and we return the user is an admin or owner only if the if statements are satisfied)