Add post deployment migration to complete pages metadata migration

Related to https://gitlab.com/gitlab-org/gitlab/issues/28781.

changelogs/unreleased/complete-pages-metadata-migration.yml

+---
+title: Add post deployment migration to complete pages metadata migration
+merge_request: 19928
+author:
+type: other

db/post_migrate/20191112023159_complete_pages_metadata_migration.rb

+# frozen_string_literal: true
+
+class CompletePagesMetadataMigration < ActiveRecord::Migration[5.2]
+  DOWNTIME = false
+
+  def up
+    Gitlab::BackgroundMigration.steal('MigratePagesMetadata')
+  end
+
+  def down
+    # no-op
+  end
+end