Make ManualRepository.all order manuals

- Add ManualRecord.all_by_updated_at and have the repo use it
Merge pull request #82 from alphagov/order-manuals

ManualRecord.all orders response by updated_at
Fix bug with Preview

The preview function had an issue with working on manuals and new
specialist documents. This commit fixes the 2 related bugs and refactors
the preview code to be more reusable between different models.