Old URLs without a language code in the URL no longer work

[ExplodingCabbage]

This is one of the two major types of broken link visible in the table I made at #2332 (comment). (The other is #2338.) Per what I see in the Wayback Machine, prior to the recent redesign these URLs redirected to the /en/ version, which seems sensible.

[ExplodingCabbage]

(For instance, http://expressjs.com/starter/static-files.html should redirect to https://expressjs.com/en/starter/static-files/)