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.

app/logical/concerns/has_dtext_links.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module HasDtextLinks
+  extend ActiveSupport::Concern
+
+  class_methods do
+    # Declare a field that has DText links. Any wiki links or external links
+    # contained in this field will be saved in the dtext_links table whenever
+    # the field is updated. This allows finding wiki pages, forum posts, and
+    # pool descriptions linking to a given tag.
+    #
+    # @param attribute [Symbol] the name of the DText field.
+    def has_dtext_links(attribute)
+      has_many :dtext_links, as: :model, dependent: :destroy
+      before_save :update_dtext_links, if: :dtext_links_changed?
+
+      define_method(:dtext_links_changed?) do
+        attribute_changed?(attribute) && DText.dtext_links_differ?(self[attribute], attribute_was(attribute))
+      end
+
+      define_method(:update_dtext_links) do
+        self.dtext_links = DtextLink.new_from_dtext(self[attribute])
+      end
+    end
+
+    # Return pages (e.g. wikis, forum posts, pool descriptions) that link to the given wiki page.
+    def linked_to(title)
+      where(dtext_links: DtextLink.where(model_type: name).wiki_link.where(link_target: WikiPage.normalize_title(title)))
+    end
+
+    # Return pages that don't link to the given wiki page.
+    def not_linked_to(title)
+      where.not(dtext_links: DtextLink.where(model_type: name).wiki_link.where(link_target: WikiPage.normalize_title(title)))
+    end
+  end
+end