posts: fix timeouts for searches using sequential navigation.

Fix certain searches timing out when using sequential navigation (page=b1234).

The problem was that the so-called "small search optimization" (AKA: force Postgres
to use the tag index for small searches instead a sequential scan) wasn't triggering
because the ORDER BY clause for sequential navigation was `posts.id desc`, and we
were only checking for `posts.id DESC`.

app/logical/pagination_extension.rb

@@ -64,11 +64,11 @@ module PaginationExtension
   end
 
   def paginate_sequential_before(before_id, limit)
-    where("#{table_name}.id < ?", before_id).reorder("#{table_name}.id desc").limit(limit + 1)
+    where("#{table_name}.id < ?", before_id).reorder("#{table_name}.id DESC").limit(limit + 1)
   end
 
   def paginate_sequential_after(after_id, limit)
-    where("#{table_name}.id > ?", after_id).reorder("#{table_name}.id asc").limit(limit + 1)
+    where("#{table_name}.id > ?", after_id).reorder("#{table_name}.id ASC").limit(limit + 1)
   end
 
   def paginate_numbered(page, limit)