Add Banzai filter for detecting potential iframes

What does this MR do and why?

Adds Banzai filter iFrameLinkFitler to detect possible iframes and tags them with the class js-render-iframe, for future a frontend MR to render. This is similar to how we handle mermaid diagrams in mermaid_filter.rb, where js-render-mermaid is added and then rendered on the frontend.

Adds the allow_iframes_in_markdown feature flag

References

Related to

• Draft: Allow iFrame embeds in markdown from all... (!200864)
• IFrame embedding does not work (#282443)

How to set up and validate locally

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