Don't add disqus comments on *all* "index" pages (including tag pages)

Spotted it at @Puuhinen's blog: disqus comments were added at tag pages. I believe it wasn't intentional (but feel free to correct me, since my site doesn't have any comments at all).

Let's use "$index" instead - it's the third parameter to `create_html_page()` function and is "yes" only when making pages for individual blog posts.