Use real URL, taking care of default pages

Actually just one fix in opengraph_meta.pt

        <meta property="og:url" content="URL of this object"
              tal:attributes="content context/@@plone_context_state/canonical_object_url" />

Using the /canonical_object_url instead of Zope's absolute_url in order to handle "default pages" of folders (e.g. I want to share /events not /events/aggregator)