This makes React UI URLs look nicer than the previous
/static/graph-new/app.html, but internally still serves all React UI
files from the compiled-in static assets directory.
Also, to allow future usage of the React / Reach router, we need to
serve the main React app's index.html on certain sub-paths that
correspond to current Prometheus's UI pages, instead of trying to serve
actual files that match the provided path name.
Signed-off-by: Julius Volz <julius.volz@gmail.com>