Ignoring upgrades for now.
We are now going to bank on the idea that upgrades will happen on the plone side before the database migration.
New legacy data that includes all cnxml 0.7 versioned data and cnxml_auto_generated.cnxml files.
Fixes tests to work with the new data.
Fixes link and img reference urls.
Move some logic around and change the StringIO usage to BytesIO.
Integrate the reference fix in the module to html logic.