Uncomment old topics page CSS and add main class hack

We'll probably want to keep the old topics page in its current form
around somewhere, as its a nice compact way to view things, like a
sitemap. No telling where that will be yet, but may as well uncomment
the CSS.

Also, the container for the new topics page should really be full-width
on mobile, so we add a gross layout hack to enable that. This should
actually probably be the case for all pages, but its limited to the
topics page for now.
Fix navigation dropdown links

This links the dropdown to the correct sections on the new topics page.
I ran into the weirdest error while working on this: for example,
linking to /sexinfo/category#the_body is fine, whereas
/sexinfo/category/#the_body (note the trailing slash) totally breaks the
images with a ton of 404s. This can also be seen just by browsing to
/sexinfo/category and /sexinfo/category/. I think this is something
weird with base URIs and .htaccess rules, but I don't care to
investigate, and we've safe for now.