Having two views for bundles (public and non-public) can cause confusion
when bundle owners want to share a URL; it's not obvious that the
private URL isn't shareable.

This change removes the private URLs, and puts all bundles under the
/bundle/<username>/<bundlename>/ URL space. For non-public bundles, this
will just 404 for non-owners.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>