Combine source_xml and tagged_text

[cmc333333]

When creating XML nodes, we generally set two, xml-containing fields: source_xml is an lxml node and tagged_text is an unescaped XML fragment. Different parts of the application use different portions of that data. Let's get rid of this duplication and confusion by standardizing on a single string field (and perhaps a cache-able XML-generating property).

Some complications:

• tagged_text is very similar to text, including the use of unescaped &s. This similarity allows layers to inspect the tagged_text and apply the results to the text field.
• Nodes may arise from an amalgamation of multiple XML elements
• tagged_text currently strips the outer-most tag (generally a P)