dtext links: add table for tracking links between wikis.

Add a dtext_links table for tracking links between wiki pages. This is
to allow for broken link detection and "what links here" searches, among
other uses.

app/models/dtext_link.rb

@@ -0,0 +1,27 @@
+class DtextLink < ApplicationRecord
+  belongs_to :model, polymorphic: true
+  enum link_type: [:wiki_link, :external_link]
+
+  before_validation :normalize_link_target
+  validates :link_target, uniqueness: { scope: [:model_type, :model_id] }
+
+  def self.new_from_dtext(dtext)
+    links = []
+
+    links += DText.parse_wiki_titles(dtext).map do |link|
+      DtextLink.new(link_type: :wiki_link, link_target: link)
+    end
+
+    links += DText.parse_external_links(dtext).map do |link|
+      DtextLink.new(link_type: :external_link, link_target: link)
+    end
+
+    links
+  end
+
+  def normalize_link_target
+    if wiki_link?
+      self.link_target = WikiPage.normalize_title(link_target)
+    end
+  end
+end