Changing legacy configuration keys should involve a migration pattern

[jfroelich]

Currently the code naively iterates over everything and does a ton of wasted processing. Moreover, it is getting more difficult to track which changes were made, when those changes were made, etc. It would be better to instead adopt an approach similar to the upgrade-needed handler in indexedDB where changes happen based on a version increment. We have the extension version readily available to us. It will require a version bump per settings schema change but maybe that is appropriate?