Show '0 days since transition' on the day

Instead of waiting until the next day to update this dashboard
message, change 'until' to 'since' as soon as the site appears to
be live from DNS details, for transition day happiness.
Remove blank lines for consistency
Make big number message work for indeterminate sites

This will make the number look odd for days just after transition
if the site's launch date is still in the future, but that's fairly
unlikely to happen, and we have a separate story for recording the
transition date more accurately rather than relying on the imported
date.
Minor refactor for dubious clarity
Merge pull request #178 from alphagov/site_launched_today_since

Show '0 days since transition' on transition day when site is live