This function sets axios's default headers to be included before all queries.
We need to call `setUserToken` in _app.tsx so that the defaults are set before every client-side call, and we also need to call it in every instance of `setServerSideProps`.
As a result, wherever we use `getCookies` and construct a `headers` object, we can remove it. Right now, we've only removed it on the top-level pages.
