Add confidential attribute to notes

- confidential s safe to export

changelogs/unreleased/207468-note-confidential-attribute.yml

+---
+title: Add confidential attribute to notes table
+merge_request:
+author:
+type: other

db/migrate/20200222055543_add_confidential_to_note.rb

+# frozen_string_literal: true
+class AddConfidentialToNote < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    with_lock_retries do
+      add_column :notes, :confidential, :boolean
+    end
+  end
+end

db/schema.rb

@@ -2805,6 +2805,7 @@ ActiveRecord::Schema.define(version: 2020_02_24_163804) do
     t.text "change_position"
     t.boolean "resolved_by_push"
     t.bigint "review_id"
+    t.boolean "confidential"
     t.index ["author_id"], name: "index_notes_on_author_id"
     t.index ["commit_id"], name: "index_notes_on_commit_id"
     t.index ["created_at"], name: "index_notes_on_created_at"

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -72,6 +72,7 @@ Note:
 - resolved_by_push
 - discussion_id
 - original_discussion_id
+- confidential
 LabelLink:
 - id
 - target_type