Add ability to return to previous BookServer Explorer App page from BookReader

Forwarded from https://github.com/ArchiveLabs/pyopds2_openlibrary/issues/26 via @MarcCoquand:

> I think if referrer is reader or openlibrary, there should be some way to return a user to a specific page when returning a book. Right now when browsing, the flow breaks because the user is sent back to the publication, rather than the book they were reading.