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/pool.rb

@@ -112,16 +112,20 @@ class Pool < ApplicationRecord
     normalize_name(name).mb_chars.downcase
   end
 
-  def self.find_by_name(name)
+  def self.named(name)
     if name =~ /^\d+$/
-      where("pools.id = ?", name.to_i).first
+      where("pools.id = ?", name.to_i)
     elsif name
-      where_ilike(:name, normalize_name_for_search(name)).first
+      where_ilike(:name, normalize_name_for_search(name))
     else
       nil
     end
   end
 
+  def self.find_by_name(name)
+    named(name).try(:first)
+  end
+
   def versions
     if PoolArchive.enabled?
       PoolArchive.where("pool_id = ?", id).order("id asc")