Create or update the breadcrumbs in the site administration
pages where it is required.
Highlight the corresponding site adminstration tab.
Highlight the primary nav to Site administration when user
is navigating to any of the site administration pages.
Also changed the boostnavbar so that the nodes in the secondary
navigation are not shown in the breadcrumbs when user is in site
administration page.
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.