Fix WYSIWYM XHTML output client-side rather than server-side

[namedgraph]

This involves wrapping the XHTML output into <div xmlns="http://www.w3.org/1999/xhtml"></div> and then canonicalizing the XML.
Doing it on the client-side would allow to get rid of 3 request interceptors:

• RDFPostCleanupInterceptor
• UpdateRequestCleanupInterceptor
• MultipartRDFPostCleanupFilter

[namedgraph]

There is a Javascript XML c14n library that could be used: https://www.npmjs.com/package/xml-c14n