Origin in per domain rules

[louix]

Part of #397

Still needs tests, and a way to migrate "Different from Tab Domain" style preferences.

Broad approach would be changing the setting to always, and adding the target domain to the ignored list, but there may be a better way.

Also need to migrate domain -> targetDomain in preferences.

[stoically]

As discussed on matrix: Nice direction, I think being able to configure origins for rules would already help a lot. Though at least one test and a working migration would be needed to progress here.