Use multi-frame webp instead of pdf for multi-page scans

[cilynx]

https://stackoverflow.com/questions/67539377/how-to-check-if-a-webp-file-is-a-gif-or-image-in-python-using-pil

[cilynx]

Compressed tiff wound up making more sense for a couple reasons:

• The default gnome tools treat a multi-frame tiff like a document -- you can scroll through the pages -- and a multi-frame webp as an animation -- opens in the browser and frames cycle through.
• Tesseract can process multi-frame tiffs natively

Addressed in 5a19c1d93c6bca7c59832ee99635ad1b1b4489c1 and the next couple after that.