Derive Truth VCF for stats calculation

This fixes #251, deriving truth VCFs for calculation of TFPN stats on
the Examine page. If a validation VCF exists for a given dataset, the
first one returned by a query for such a VCF is inferred and used.

This fixes #267, so truth VCFs can be submitted.

This adds a badge to the runs table on truth VCFs, indicating to the
user that the VCF displayed there is a validation VCF.
So they're technically URI references, since they don't indicate the scheme. When we add support for local files/files on the server native FS, we may want to enforce adding them scheme, but it seems superfluous for now.