Use select_related to get versioning stuff for WidgyPages

When rendering a WidgyPage, there are 3 extra queries -- for
the VersionTracker, VersionTracker.head, and then, usually,
VersionTracker.head.root_node. We can use select_related to get rid of
these queries. VersionTracker.head.root_node is speculative, but is
needed most of the time.