[DashAuth] OAuth redirect JS does not work in IE (requires fetch polyfill)

[davecap]

The OAuth JS code that handles Dash auth redirection uses window.fetch which requires a polyfill for some browsers.

I started the following PR by including the https://github.com/github/fetch polyfill, but it doesn't quite work yet. https://github.com/solvebio/solvebio-python/pull/226