Re-add ignore_column for import columns

This `ignore_column` was present for a while but recently removed, but
to ensure we don't get error 500s let's keep it for a while.

app/models/project.rb

@@ -55,6 +55,8 @@ class Project < ApplicationRecord
   VALID_MIRROR_PORTS = [22, 80, 443].freeze
   VALID_MIRROR_PROTOCOLS = %w(http https ssh git).freeze
 
+  ignore_column :import_status, :import_jid, :import_error
+
   cache_markdown_field :description, pipeline: :description
 
   delegate :feature_available?, :builds_enabled?, :wiki_enabled?,