[WIP] add support for packaging with PDF.js extension - try 4

This PR is a blend of https://github.com/hypothesis/h/pull/1433 and https://github.com/hypothesis/h/pull/1514.

Here is how it works, and what it contains:
 - It bundles the static (generic) build of PDF.js
   (Just as in @tilgovi's PR, except that Iused	the latest version)
 - It has a single added line to viewer.html, to load our code
   (Same as in both PRs.)
 - It relies on @tilgovi's refactored embed code (using a separate
   bootstrap script, instead of inline code), which is already
   on the master branch, so the `eval()` based script execution
   workarond is not longer necessary.
 - The latest version of Annotator's PDF plugin is already on master,
   so it's not included	in this	PR.
 - For PDF document, we	don't load the document	plugin;	instead,
   we rely on the PDF plugin for url+metadata extraction
   (This comes from my PR, but was also	copied to @tilgovi's)
 - When	displaying annotations,	avoid non-clickable URIs,
   like the ones based	on PDF fingerprint. (Adapted from my PR)

More to	come:
 - required changes in background.js
 - required changes in manifest.json
 - testing


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