Do not regenerate the avif file each time it is requested

If this is merged after 🐛 Distinguish between two files have the same name but a different extension Closed: duplicate , the MR will need to be updated so that Avif::getAvifCopy() uses $destination = $this->getAvifDestination($uri, '@directory@filename.@extension.avif'); to get the destination.

I've merged this MR in to the MR for 📌 Test coverage Needs review and started refactoring a little bit. Needs work for the test coverage, have added some @todo comments.

This has been merged. Another DDoS risk mitigation!

Automatically closed - issue fixed for 2 weeks with no activity.