Clear existing matching items when new exclusion rule is add

[junkblocker]

When a new exclusion rule is added, existing items matching the new rule should be added. This probably should be done via some sort of preview in case there are typos in the regex/rule or user is not aware of the extent of items it would clear.

[ghost]

Should the content from the now-excluded website be deleted, or just hidden? If hidden, the content for the site could re-appear when the exclusion rule gets removed. Personally, I think this makes more sense for the exclusion. If the user wants the context deleted, they should use the delete history function.

[junkblocker]

@morris-jason , sounds good to me.