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)

app/logical/post_query_builder.rb

@@ -542,7 +542,7 @@ class PostQueryBuilder
   # better.
   def optimize_search(relation, small_search_threshold)
     return relation unless small_search_threshold.present?
-    return relation unless relation.order_values == ["posts.id DESC"]
+    return relation unless relation.order_values.map(&:downcase).in?([["posts.id desc"], ["posts.id asc"]])
 
     if post_count.nil?
       # If post_count is nil, then the search took too long to count and we don't