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

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.

Removed requiremodintro setting from all core activity plugins and replace
with a single global setting.
Deprecated moodleform_mod::add_intro_editor and replaced with
moodleform_mod::standard_intro_elements

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.

To prevent ugly DB errors.