Tag cloud results should not display versions

Currently the tag cloud results display live documents, proxies and versions, which makes a lot of noise. It should display only live documents and proxies.

As a consequence, to make everything consistent, we should also not display tags that exist only on versions in the tag cloud, since clicking them wouldn't display any document.
And we shouldn't take the versions into account for the tag's size in the cloud (currently, tagging a live document makes the tag quite big in the could since we actually tag the live document AND the version).