If PDF.js is launched with a `file` query-string parameter that is a
relative URL, then `window.PDFViewerApplication.url` will be that
relative URL.

When capturing the metadata from the PDF in order to search for
annotations or find relevant groups, we want to use an absolute URL.
This commit uses the existing `normalizeURI` helper to resolve the
possibly-relative URL.

Fixes https://github.com/hypothesis/product-backlog/issues/579