Co-authored-by: Sujith Haridasan <sujith@moodle.com>

Make the book navigation more distinguishable from activity navigation

* Users who has permission should be able to view after turning editing off

This adds the details of the next/previous chapter to the image
attributes and link for the in-page chapter-to-chapter navigation.

dirname() is a slow function compared with __DIR__ and using
'/../'.  Moodle has a large number of legacy files that are included
each time a page loads and is not able to use an autoloader as it is
functional code.  This allows those required includes to perform as
best as possible in this situation.

Based on a patch by Matthew Cannings, this replaces Book's previous/next arrow
image links with a choice of: no links, image links, or chapter title text links.

This reverts commit ab7632b7.