Serve index.php as HTML

[Chris Warburton]

Looks like darkhttpd is choosing to serve up index.php (which is
actually a HTML file) as a download, rather than displaying the content.

This breaks some of our historical redirects, so it would be nice to
fix.

The `--mimetypes <filename>` option looks like one way to fix this. From
https://github.com/andreasgal/darkhttpd:

> Add mimetypes - in this case, serve .dat files as text/plain:
>     $ cat extramime
>     text/plain	dat
>     $ ./darkhttpd /var/www/htdocs --mimetypes extramime

A possible fix might be to write a HTML file, e.g. index-redirect.html,
and symlink index.php to that file. That would be cleaner, but may not
work.