Add <link rel="prev"> and <link rel="next"> tags to the forum topic show
page, the pool show page, and the favgroup show page.
This is kind of useless now since Google hasn't used these tags for SEO
for years ([1]), and userscripts can use `#paginator-next` and
`#paginator-prev` to find the next/prev pages, but whatever.
[1]: https://yoast.com/google-doesnt-use-rel-prev-next-for-pagination
