Bug: when a search timed out we got the generic failbooru page instead
of the search timeout error page.
Cause: when rendering the <link rel="next"> / <link rel="prev"> tags in
the header, we may need to evaluate the search to determine the next or
previous page, but if the searches times out then this fails, which
caused Rails to throw a ActionView::Template::Error because an exception
was thrown while rendering the template.
Likewise, rendering the attributes for the <body> tag could fail with an
ActionView::Template::Error because the call to `current_item.present?`
forced evaluation of the search.
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
