Use content_view_gui to preview uploads

If the uploaded content's publishing workflow is interrupted,
either because of approval, asynchronous publishing, or any
interruption based workflow, there is no node to run node_view_gui on.

If the object has a node, a link to the main node will be used. If it
doesn't, no link will be made.

Existing overrides of node/view/line_thumbnail.tpl must be updated
to override content/view/line_thumnail.tpl instead.