Add native PDF conversion support for OS X / Windows

[joeflack4]

Description

When testing locally on Mac OS X or Windows, converting to PDF throws an exception and crashes the app.

Inspecting wkhtmltopdf on OS X, for example, reveals:

webui/bin/wkhtmltopdf: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.18, BuildID[sha1]=b6566a9e44c43a0eebf18d8c1dc6cb616801a77e, stripped

There are other downloads available for other systems on the website: https://wkhtmltopdf.org/downloads.html

I put some in ppp_web/bin/installers. But I do not know if it is possible to make a locally executable binary for OS-X. Other possible solutions:

• Install globally on system? Hmm...
• Docker? But this would increase development complexity.

[joeflack4]

@bciar After finishing #19, we can talk about this one soon. Maybe this one is easy. Maybe it is very hard. I don't know.