Allow groups to provide default values for its checks

Currently, the way a check group works is that we can either enable or disable setting overrides. For example, if we enable location overrides, then all the checks within that group will use that location. If we disable location overrides, then the individual check location applies.

It would be helpful if check groups provided default values. For example, if both the check and its group have a location configured, then we use the check’s location. If the check does not have a location, then we default to the group.