Fix #1883: Add <link> elements for all paginated pages.

Add <link rel="prev"> and <link rel="next"> elements to most pages with
pagination. This should work on all index pages, but it won't work for
things like pool or forum topic show pages.

Also remove the <link rel="top"> element (wasn't useful, was just a link
back to the root url).

app/logical/pagination_extension.rb

@@ -60,6 +60,30 @@ module PaginationExtension
     end
   end
 
+  def prev_page
+    return nil if is_first_page?
+
+    if paginator_mode == :numbered
+      current_page - 1
+    elsif paginator_mode == :sequential_before
+      "a#{records.first.id}"
+    elsif paginator_mode == :sequential_after
+      "b#{records.last.id}"
+    end
+  end
+
+  def next_page
+    return nil if is_last_page?
+
+    if paginator_mode == :numbered
+      current_page + 1
+    elsif paginator_mode == :sequential_before
+      "b#{records.last.id}"
+    elsif paginator_mode == :sequential_after
+      "a#{records.first.id}"
+    end
+  end
+
   # XXX Hack: in sequential pagination we fetch one more record than we
   # need so that we can tell when we're on the first or last page. Here
   # we override a rails internal method to discard that extra record. See