pools: stop using the pool_string field (#4160).

Stop using the pool_string field internally, but keep maintaining it
until we can drop it later.

* Stop using the pool_string for `pool:<name>` metatag searches.
* Stop using the pool_string in the `Post#pools` method. This is used to
  get the list of pools on post show pages.

app/models/post.rb

@@ -1020,11 +1020,7 @@ class Post < ApplicationRecord
 
   module PoolMethods
     def pools
-      @pools ||= begin
-        return Pool.none if pool_string.blank?
-        pool_ids = pool_string.scan(/\d+/)
-        Pool.where(id: pool_ids).series_first
-      end
+      Pool.where("pools.post_ids && array[?]", id).series_first
     end
 
     def has_active_pools?