Introduce an `acts_as_loofah` method #277

Job	Run time
rubocop	12s
downstream-rails-html-sanitizer	22s
test (2.5)	21s
test (2.6)	24s
test (2.7)	24s
test (3.0)	11s
test (3.1)	8s
test (3.2)	10s
test (3.3)	11s
test (3.4)	11s
test (head)	7s
test (truffleruby-head)	36s
test (jruby-9.4)	39s
	3m 56s

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