pools: add ability to search for pools linking to given tag.

Add ability to search for pools linking to a given tag in the pool
description. Example:

    https://danbooru.donmai.us/pools?search[linked_to]=touhou

(This isn't actually exposed in the UI to avoid cluttering the pool
search form with rarely used options.)

Pools with broken links can be found here:

    https://danbooru.donmai.us/dtext_links?search[has_linked_tag]=No&search[has_linked_wiki]=No&search[model_type]=Pool

Lays the groundwork for fixing #4629.

script/fixes/090_create_pool_dtext_links.rb

@@ -0,0 +1,9 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+with_confirmation do
+  Pool.find_each do |pool|
+    pool.update_dtext_links
+  end
+end