[#10] Improve harvested metadata API

Some improvements on the endpoints that return the contents of the
harvest objects:

* Nicer URLs with redirects to the old ones
* Returning the raw harvest object content is available on the main
 harvest extension, so just redirect there
* Support for showing the original document of a harvest object, if
 present
* Suport for defining a custom XSLT for the HTML view, via

ckanext.spatial.harvest.xslt_html_content
ckanext.spatial.harvest.xslt_html_content_original