Replace <img> with PDF display

[choilmto]

See pdf.worker.js.

[jpetso]

Related: Electron Forge is working towards their 6.0 release - see e.g. electron/forge#621, https://v6.electronforge.io/cli - and is replacing the deprecated electron-compile package with webpack as default configuration. Best feature is that it's actually documented how to set up webpack now (as a JS noob, I've been struggling to understand how the pieces come together).

Once Electron Forge 6.0 is out, we should switch over, update all of our dependencies accordingly, use webpack and thus get proper support for pdf.worker.js via ES6 imports.

We can also consider upgrading to 6.0 and webpack earlier than the actual release, but I wonder whether that'll be more trouble than just waiting for a bit longer.