Add domain parameter to theme cookie

This adds the domain parameter to the cookie we're setting for the themes, which should make the cookie persist across subdomains. I initially thought of just adding domain=tildes.net but thought that might not work well with the localhost version and people who'd want to fork the website in the future. So document.location.hostname seemed the smartest thing to do.

This would fix #27 (closed) on this side of Tildes however new styles would have to be made over at tildes-static-sites, I've made an issue for it: #9.