Fix sanitizer making block level elements unreadable (#10836)

Fix #10834

app/lib/sanitize_config.rb

@@ -19,6 +19,20 @@ class Sanitize
       node['class'] = class_list.join(' ')
     end
 
+    UNSUPPORTED_ELEMENTS_TRANSFORMER = lambda do |env|
+      return unless %w(h1 h2 h3 h4 h5 h6 blockquote pre ul ol li).include?(env[:node_name])
+
+      case env[:node_name]
+      when 'li'
+        env[:node].traverse do |node|
+          node.add_next_sibling('<br>') if node.next_sibling
+          node.replace(node.children) unless node.text?
+        end
+      else
+        env[:node].name = 'p'
+      end
+    end
+
     MASTODON_STRICT ||= freeze_config(
       elements: %w(p br span a),
 
@@ -40,6 +54,7 @@ class Sanitize
 
       transformers: [
         CLASS_WHITELIST_TRANSFORMER,
+        UNSUPPORTED_ELEMENTS_TRANSFORMER,
       ]
     )