PDF preview from Keybase.pub do not work on chrome

Keybase.pub/[username] page seems to try to load PDF files from [username].keybase.pub into an iframe to preview it , but [username].keybase.pub `x-frame-options` header with `ALLOW-FROM` is [not available on chrome or safari](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options)

i haven't checked much but it may be able to be fixed by using `Content-Security-Policy`, with `frame-ancestors` and [that should work for chrome,firefox,edge and safari](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors). And if compatiblity is needed, i believe both headers can be set and `x-frame-options` ill just get ignored if `Content-Security-Policy ` is set and compatible

Start by locating the Keybase.pub PDF preview path and checking the response headers from [username].keybase.pub in Chrome and Safari. Confirm the intended header behavior and verify that PDF files load in the iframe across the affected browsers.

Written by the indexing model from the issue text.