Add per-site configuration for which target to use for preferred entry setting

[luckyrat]

Label: "Preferred entry setting applies to : " Domain, hostname, page (dropdown / radio buttons)

Explanatory note: “Only affects new preferences. You can change existing preferences by choosing a new matched entry next time you visit the site.”

Need to watch out for conflicts with upcoming work on replacing the current options page with a VueJS approach.

We can end up with multiple records for each web page - e.g. page, hostname or domain. In theory, advanced users could hack into the underlying config data to set up even more complex systems involving prefix and regex methods too. We must therefore plan carefully how to manage this situation for users. The main question is whether to delete any existing configurations when an entry is marked as the preferred entry for a specific URL. Deleting always could prevent some advanced configurations such as a default entry for the entire domain but overriding that choice for one or more specific hostnames. Not deleting will lead to a proliferation of hidden useless data in the configuration and no doubt some complicated steps for users to complete in order to achieve certain desired reconfigurations. I think the compromise outlined in feature 1 will minimise potential confusion and at least allow for a “reset” by deselecting and selecting the preferred entry on each related site if any advanced configuration or changes to the PSL cause problems in future.