Improved XSLT support in content XML cache

Optionally serialize JSON property data as XML to the content XML cache, to improve the support for XSLT renderings.
This option is enabled in umbracoSettings.config by setting the value of ```<PublishJsonAsXml>``` to ```true``` (default ```false```).
Custom property value editors can still override this behavior by implementing the ConvertDbToXml method.
XML serialized JSON property data is converted back to JSON for use in property value converters, so standard rendering using property value converters with razor templates is still possible even with this option enabled.