WIP: add support for packaging with PDF.js extension - try 3

This a further alternate version of #1432, simplified even further.

Changes compared to try 2:
 * Completely dropped the contentscript. (Removed it from manifest, so there is no need to disable the functionality shipped in it; hence I could also remove the overlay file itself.)
 * Completely dropped relying on pdfHandler.js's PDF detection features; replaced it with a simple filename detection. This means we no longer need to track PDF state for tabs. Since we have already stopped relying on the interception features of pdfHandler.js in try2, this means that we don't need pdfHandler.js anymore *at all* - hence I could remove it, both the overlay file from the repo, and the reference from manifest.js
 * I also dropped some more scripts from manifest, which we don't need: 
   * executeScriptInFrame.js
   * pdfHandler-v2.js
   * pdfHandler-vcros.js

... which means that now we have only two remaining overlay files:
 * extension-router.js
 * viewer.html

... and both with truly [minimal](https://github.com/hypothesis/h/commit/fb439cec178407eef4d3fe00cb3bc383ed3ba896#diff-1) [changes](https://github.com/hypothesis/h/commit/fb439cec178407eef4d3fe00cb3bc383ed3ba896#diff-2).

What would be a better name for this. It's not accurate to call it `inPDFjs` because it's actually testing that we're in our own extension.