Use strong_memoize to customize the SanitizationFilter whitelist

I never liked the hacky `customized?` method anyway, so this is cleaner.

lib/banzai/filter/sanitization_filter.rb

@@ -4,27 +4,20 @@ module Banzai
     #
     # Extends HTML::Pipeline::SanitizationFilter with a custom whitelist.
     class SanitizationFilter < HTML::Pipeline::SanitizationFilter
+      include Gitlab::Utils::StrongMemoize
+
       UNSAFE_PROTOCOLS = %w(data javascript vbscript).freeze
       TABLE_ALIGNMENT_PATTERN = /text-align: (?<alignment>center|left|right)/
 
       def whitelist
-        whitelist = super.dup
-
-        customize_whitelist(whitelist)
-
-        whitelist
+        strong_memoize(:whitelist) do
+          customize_whitelist(super.dup)
+        end
       end
 
       private
 
-      def customized?(transformers)
-        transformers.last.source_location[0] == __FILE__
-      end
-
       def customize_whitelist(whitelist)
-        # Only push these customizations once
-        return if customized?(whitelist[:transformers])
-
         # Allow table alignment; we whitelist specific text-align values in a
         # transformer below
         whitelist[:attributes]['th'] = %w(style)