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.

db/migrate/20191023191749_create_dtext_links.rb

@@ -0,0 +1,13 @@
+class CreateDtextLinks < ActiveRecord::Migration[6.0]
+  def change
+    create_table :dtext_links do |t|
+      t.timestamps
+      t.references :model, polymorphic: true, null: false
+      t.integer :link_type, null: false
+      t.string :link_target, null: false
+
+      t.index :link_type
+      t.index :link_target, opclass: "text_pattern_ops"
+    end
+  end
+end