Revert "DEV: Remove stale ignored_columns from models."

This reverts commit 9f5c8644d0.

Have to revert because the ignored columns have not been dropped.

app/models/embeddable_host.rb

@@ -10,6 +10,9 @@ class EmbeddableHost < ActiveRecord::Base
     self.host.sub!(/\/.*$/, '')
   end
 
+  # TODO(2021-07-23): Remove
+  self.ignored_columns = ["path_whitelist"]
+
   def self.record_for_url(uri)
 
     if uri.is_a?(String)