Remove callback to sort parts before saving travel advice

There are issues with how Mongoid handles embedded documents
which does not make this work as intended. After discussion,
instead of fighting Mongoid, we're going to go with a non-default
scope for parts sorted in the correct order.
Allow parts to be sorted by the order value

Use a scope to sort parts by the value of the order
attribute.
Merge remote branch 'origin/use-scope-to-order-parts'