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.

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.

Rather than the generic "sans-serif"

Also add an /about page with licensing info for the icons, as well as
links to their sources, and other random trivia and nerdery.