Allow loading PDFs from websites blocking cross-origin requests

[pietroalbini]

For security reasons, browsers blocks XHR requests to other origins (websites) if those don't explicitly allow this with the Access-Control-Allow-Origin HTTP header. Unfortunately, this means Persuade isn't able to load slides from most of the websites.

The only workaround I think would work is creating a transparent proxy returning the requested PDFs and host it somewhere, so Persuade can load the slides from there.