Support PDFs

[casey]

Someone inscribed the bitcoin whitepaper, so we should support PDFs:

https://ordinals.com/content/85b10531435304cbe47d268106b58b57a4416c76573d4b50fa544432597ad670i0

[casey]

PDFs may be hard to support:

whatwg/html#3958
https://bugs.chromium.org/p/chromium/issues/detail?id=413851

Chrome renders PDFs with a plugin, and plugins aren't allowed inside sandboxed iframes. Inscription previews are all iframes, so we would have to un-sandbox iframes containing PDFs. This would suck, since it breaks our current model of not needing to know anything about inscriptions to render them, just instantiate an iframe with the right /preview URL.

PDFs are also pretty bonkers in general. They can contain scripts, and do other wacky things. Our best option may be to use some kind of PDF preview service, or render them using JS.

[casey]

The way to do this is probably https://github.com/mozilla/pdf.js.

[casey]

It is done, and it wasn't even that bad!