FEATURE: Whitelist the `allowfullscreen` iframe attribute (PR #8086)

Adding this to core allows showing the fullscreen button on whitelisted, external iframe videos.

Example from Vimeo

iframe

GitHub

You’ve signed the CLA, rishabhnambiar. Thank you! This pull request is ready for review.

This seems fine but I am curious why you created a new file rather than adding it to white-lister.js.es6?

Ahhh, I was looking for an existing file in ../discourse-markdown/ for some reason. TIL about white-lister.js.es6, much better thanks!