Leverage server-side HEIC conversion

[swissspidy]

Related:

• Evaluate libheif alternative #483

If WordPress/wordpress-develop#7034 lands and no client-side HEIC conversion is possible, it could be at least done on the server. The thumbnails could still be generated client-side though.

It would make things a bit more complicated:

1. Upload HEIC image to the server as a regular attachment
   • Let WP do the conversion server-side
   • Do not create any thumbnails though
2. Get back the converted JPEG on the client
3. Create all thumbnails on the client and sideload them