Enhancements to links between different versions of the same dataset

Following up from #57, we should also consider the case where a dataset has more than two versions. This hasn't come up yet, but there's a chance it will.

In the current system "middle" versions will both have a notice that a new version is available and a link to the previous one at the bottom. This will require the user to follow the links up the chain to the newest version.
It would be preferable to link to the newest version from all older pages and it would also be nice if the latest version listed all previous versions. If we want to do this, we'd have to make some more changes so that the landing page creation function follows the chain of references to link the latest one (if it's old) or list all available versions (if it's the newest).