Fix/cdodge/incorrect html serialization

@cpennington @ormsbee @cahrens can one of you review?

We got a bug report [STUD-676] where someone tried to export an HTML module like the one in with_styling.html (test file). Basically the HTML definition_to_xml() first tries to just treat the HTML as an XML block, but that causes all of the stuff in the 'p' HTML tag to get dropped in the export.

EDIT: This is also the same problem for STUD-677 where the HTML block was just a simple &lt;img src='...' /&gt;

My fix is to always write HTML content into a separate .html file