Add PDFjs specific highlighting to support latest release

This removes the alpha channel from highlights in PDFjs so that the appear visible when rendered inside a container with `opacity: 0.2` set. I've isolated this fix in a new file called `pdfjs-overrides.scss` so that others can build the assets without this "feature".

This does mean that older versions of PDFjs will now be rendered with much more vibrant highlights. This will affect FF users using an old browser version, places where h is manually embedded in an older version of PDFjs and bookmarklet users trying to inject h into an old version of PDFjs. I don't think any of these cases are too critical.

We will need to look at a more sustainable way of inserting highlights into PDFjs in future though as I can only see this breaking again in the future.