Investigate downloading PDF.js at runtime.

[Lunarequest]

Currently we use an inline version of PDF.js that is stored in app/libs/pdfjs by moving pdfjs outside the application instead shipping our own pdfjs version.

-- drawbacks
network access will always be required on first start up
on slow networks this could greatly increase download times.
this could also increase startup times on launch

-- possible mitigations
ship a cached pdfjs.zip(latest at time of build) and extract allowing the user to use nightpdf immediately.
produce a notification on a newer PDF.js and download only if the user requests
only check for new releases after launch asynchronously.

-- benfits
user always has the latest PDF.js version which often can include fixes and improvements
we can remove PDF.js from our direct source tree and move it elsewhere