racket/draw: fix problems with `set-argb-pixels` on scaled, no-alpha bitmaps

Also, fix the docs to clarify that `just-alpha?` as #t means a no-op for
a target bitmap that has an alpha channel.
web pages: add "recent" links at download-racket-lang.org

These links reflect soft links that are in place at
"mirror.racket-lang.org", but they're needed as links/rewrite on the
main site for some purposes, such as references from the tech-report
pages.
web pages: use UTC for computing month names

Otherwise, generated web pages can differ depending on which timezone
is used to render them.