Sort parts by the order attribute

Sort the list of parts by the value in the order attribute, so that changes to the order are persisted when the edit form is reloaded.

This fixes an issue with the order of parts not being reflected in the form of travel advice publisher.

There is a chance that this may break existing behaviour if there have been previous assumptions made around how parts are sorted. I'd appreciate a glance at this from @mnowster or @fatbusinessman to see if this might break the sorting of parts in Publisher or Content API.