Change delivery failure tracking to work with hostnames instead of URLs (#13437)

db/migrate/20200407201300_create_unavailable_domains.rb

@@ -0,0 +1,9 @@
+class CreateUnavailableDomains < ActiveRecord::Migration[5.2]
+  def change
+    create_table :unavailable_domains do |t|
+      t.string :domain, default: '', null: false, index: { unique: true }
+
+      t.timestamps
+    end
+  end
+end