fix some gaps which would allow the temporary xml attributes 'parent_sequential_url' and 'index_in_children_list' to get persisted in the database, whereas they are meant to be only scoped during export/import
verticals shouldn't have 'data'
Merge pull request #763 from edx/fix/cdodge/strip-transient-draft-serialization-attributes

fix some gaps which would allow the temporary xml attributes 'parent_seq...