This refactors `setUserToken` into `accountCookie`, which just returns a cookie if it exists, and `setHeaders`, which sets the Axios header defaults.
Then, we update the calls in all the required files.
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.
