Change `EPUBContentSelector` selectors captured in VS books to include
the resolved URL of the content document (eg.
"https://jigsaw.vitalsource.com/books/{book_id}/...") rather than just
the path ("/books/{book_id}/...").

This may make it easier to work with these URLs outside of the VitalSource
viewer, and should also make these URLs more consistent with
`EPUBContentSelector` selectors captured in other ebook readers in future.