It has all the benefits previously mentioned in commit 2f547251, as well
as mirroring the common programming convention of using a "_" prefix to
identify private variables/functions

Hides files beginning with _ from mod_autoindex listings

As well as 404ing on files starting with "_", also 404 on files starting
with ".", as these are also private. Except for /.well-known/, because
that's... well known, and should be public.

Because apparently green doesn't show up super-bright with traditional
CMYK printing, so my business card with the green-on-black header
wouldn't have looked great - unless I was willing to pay a lot of money
for special printing.

However, apparently yellow does show up well (probably because it's the
Y in CMYK), and because old-old-school CRTs came in amber-on-black as
well as green-on-black, switching to amber-on-black lets me keep the
same kind of aesthetic while being more print-friendly.

Also, it looks less Matrix-y. On the one hand, that's better because it
looks less derivative. On the other, it's worse, because it doesn't grab
you quite as instinctively as it did before.

I've done it with alternate stylesheets though, so you can still pick
the green colorscheme if you want.

The code is now a bit longer, but it's also a bit clearer. The resulting
HTML is also clearer.

Allows putting all the repeated stuff in one place (DRY!), so that if it
needs changing it only needs to change once.

Start the name of the included file with "@", and disallow the retrieval
of files beginning with "@" (returning 404) so that it can't be
downloaded, because it won't output anything meaningful.

Why "@"? Because it's about the only ASCII punctuation character that
doesn't mean anything special to most shells (bash), or programs, and is
therefore easy to use as a filename on the command line. I almost went
with "+", but it turns out that nvim treats arguments beginning with "+"
as options. So "@" it is.