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 commit replays, conditionally, all the upgrade steps from
MOODLE_19_STABLE to MOODLE_23_STABLE in the mod_book activity. That
guarentees that any site using the mod_book before landing to core,
no matter if it was the latest or an outdated one will upgrade
perfectly to the expected current version.
As a general rule, everytime one contrib plugin lands to core, its
complete upgrade code must be kept, at least until the core version
that introduced it, is out completely from the upgrade requirement
conditions.
In this case, with the missing upgrade code being added to 2.4, it
will be safe to delete the upgrade steps once 2.5 (or upwards) become
a requirement. Never always.
