The React app's assets are now served under /assets, while all old
custom web assets (including the ones for console templates) are now
served from /classic/static.

I tested different combinations of --web.external-url and
--web.route-prefix with proxies in front, and I couldn't find a problem
yet with the routing. Console templates also still work.

While migrating old endpoints to /classic, I noticed that /version was
being treated like a lot of the old UI pages, with readiness check
handler in front of it, etc. I kept it in /version and removed that
readiness wrapper, since it doesn't seem to be needed for that endpoint.

Signed-off-by: Julius Volz <julius.volz@gmail.com>