Fix a memory leak caused by Banzai::Filter::SanitizationFilter

What does this MR do?

It creates a dup of Banzai::Filter::SanitizationFilter whitelist transformers container to avoid a memory leak (more details in the commit message).

Are there points in the code the reviewer needs to double check?

N/A

Why was this MR needed?

Memory leaks are BAD :)

What are the relevant issue numbers?

N/A

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together