Expose auth token in response headers

[kellenmace]

Expose auth token in response headers so it can be used to update the auth token stored on the client, similar to what's shown in this Apollo link: https://github.com/benawad/slack-clone-client/blob/31_fix_auto_refresh_of_jwt_tokens/src/apollo.js#L18-L36

We are doing this already with the refresh token. This PR merely exposes the auth token, too.

Closes #98

[kellenmace]

I've released a YouTube video showing folks how to use the new authToken that comes back to update the authToken they have stored in their app, but it won't actually work until this PR gets merged :)

My Apollo Link that handles getting the x-jwt-auth header and saving it in memory for that app (along with the refresh token) is here: https://github.com/kellenmace/run-through-history-frontend/blob/main/src/services/apollo.js#L126-L151