Create and use baseline digests for each site separately

The 5.x aka: 12.x implementation of ConfigurationBaselineServiceImpl creates a single digest of baseline contents and stores it along with the baseline. It appears that the ConfigurationServiceImpl logic doesn't actually use this yet.

1. With HCM Sites, we now need to compute baselines for each extension separately and store them somehow. This will require CND changes.
2. We should actually use these digests to detect when no bootstrap diff is necessary and speed up repo launch times.