Ship the PDF.js extension inside our Chrome extension

This is what this does:

* Take our PDF.js fork (from https://github.com/csillag/pdf.js/, for-h branch)
* Build the Chrome extension from it
* Add the contents of the plugin to h/browser/chrome_pdfjs/
* Merge the contents of manifest.json from both extensions
* Modify the extension building script (h/script.py) to include these files in the extension.

  * * *

The same code is also present on the pdf-support-7 branch (which is pdf-support-6 + pdfjs-inside-chrome-extension), but I am opening a separate PR for this, because this is an orthogonal effort, and has nothing to do with our code changes.

There are a few bugs which should be solved (#892, #893), but other than that, this seems to work.
(See the [extension in the Chrome web store](https://chrome.google.com/webstore/detail/pdf%20hypothesis/bipacimpfefoidapjkknffflfpfmjdog), which was built using this code.).