Refactor TargetScrubber
Refactor TargetScrubber

TargetScrubber asts as reversal to PermitScrubber.
Two methods `allowed_node?` and `scrub_attribute?` is now
too much to know about implementaion of PermitScrubber.
It should simply reverse a result of parent method.

rails/rails-html-sanitizer,jkraemer/rails-html-sanitizer,Shopify/rails-html-sanitizer,maclover7/rails-html-sanitizer,siddhartham/rails-html-sanitizer