Support PDF uploads

[huumn]

imgproxy allows for conversion of PDFs to images, so add support for PDF uploads.

[ekzyis]

PDF only? What about epubs?

-- https://stacker.news/items/429663

Since I don't see EPUB specifically mentioned in the improxy docs¹, I think if we want to support EPUBs, we need to convert them to PDF first.

Another option is to implement native uploads. But this might be another ticket or an iteration on this ticket for PDFs with a lot of pages.

Footnotes

1. https://docs.imgproxy.net/image_formats_support ↩

[brymut]

Still working on this @Soxasora ?