Replace `axios` with `isomorphic-unfetch` to cut down bundle size

[mikestopcontinues]

Hey, I'm exploring userfront as an alternative to Auth0. It looks great! This FR isn't a dealbreaker or anything, but axios is pretty heavy now that fetch and almost complete browser support. isomorphic-unfetch fills in the gap, providing node-fetch on the server and a really thin polyfill in the browser. Cheers!

[tyrw]

@mikestopcontinues Thanks for writing this up. We've had it in mind for a little while and totally agree.

[RJFelix]

Moved to Linear DEV-759