Fix #4869: "Random" button raises exception when viewing ordfav.

Fix exception during https://danbooru.donmai.us/posts/random?tags=ordfav:nonamethanks

Before we were doing a query like this:

    SELECT
      "posts".*
    FROM
      "posts"
    INNER JOIN
      "favorites" ON "favorites"."post_id" = "posts"."id"
    WHERE
      (favorites.user_id % 100 = 64 AND favorites.user_id = 52664)
      AND "posts"."id" = 343894
    ORDER BY
      favorites.id DESC,
      posts.id DESC,
      ID=343894 DESC

but `ID=? DESC` is ambiguous during an ordfav: search because of the
join on the favorites table. The fix is to qualify the reference as
`posts.id`.

app/logical/concerns/searchable.rb

@@ -505,7 +505,7 @@ module Searchable
   def find_ordered(ids)
     order_clause = []
     ids.each do |id|
-      order_clause << sanitize_sql_array(["ID=? DESC", id])
+      order_clause << sanitize_sql_array(["#{qualified_column_for(:id)} = ? DESC", id])
     end
     where(id: ids).order(Arel.sql(order_clause.join(', ')))
   end