Fix slow Markdown rendering

The sanitize transformers were being duplicated each time the Markdown
renderer was called, leading to expontential growth in rendering times.

The problem was that although HTML::Pipeline::SanitizationFilter.WHITELIST
is a frozen hash, the `:transformers` array can be modified. We need
to do deep copy of this to avoid adding duplicates.

Closes #49409

changelogs/unreleased/sh-revert-markdown-changes.yml

+---
+title: Fix slow Markdown rendering
+merge_request: 20820
+author:
+type: performance

lib/banzai/filter/sanitization_filter.rb

@@ -13,7 +13,7 @@ module Banzai
 
       def whitelist
         strong_memoize(:whitelist) do
-          customize_whitelist(super.dup)
+          customize_whitelist(super.deep_dup)
         end
       end
 

spec/lib/banzai/filter/sanitization_filter_spec.rb

@@ -54,6 +54,18 @@ describe Banzai::Filter::SanitizationFilter do
       expect(instance.whitelist[:transformers].size).to eq control_count
     end
 
+    it 'customizes the whitelist only once for different instances' do
+      instance1 = described_class.new('Foo1')
+      instance2 = described_class.new('Foo2')
+      control_count = instance1.whitelist[:transformers].size
+
+      instance1.whitelist
+      instance2.whitelist
+
+      expect(instance1.whitelist[:transformers].size).to eq control_count
+      expect(instance2.whitelist[:transformers].size).to eq control_count
+    end
+
     it 'sanitizes `class` attribute from all elements' do
       act = %q{<pre class="code highlight white c"><code>&lt;span class="k"&gt;def&lt;/span&gt;</code></pre>}
       exp = %q{<pre><code>&lt;span class="k"&gt;def&lt;/span&gt;</code></pre>}