Optionally update sources list instead of disabling them

[Wolter HV]

With every upgrade, all custom PPA are disabled to prevent apt from running into errors when updating. However, instead of disabling the PPA it would be better to have apt (or a utility) dynamically modify the PPA installed.

First, the PPA would be checked to see if it has packages for the OS upgrade. If this is successful, then just the OS version line will be modified in the list file of the respective PPA. If the check is unsuccessful, then it must be left as was. If the PPA is not accessible at all, it should be disabled.