Add a control group to the SiteIsolationExtensionsActive.
"Control" corresponds to the Disabled group of the [non-synthetic]
SiteIsolationExtensions field trial. In practice, our analysis
has been skewed by the fact that SiteIsolationExtensionsActive/Default
includes samples from populations where the SiteIsolationExtensions trial was
not operative; these samples were a source of confounding variables.