MDL-73848 administration: Fix the breadcrumbs and nav highlights

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.

admin/plugins.php

@@ -52,6 +52,8 @@ $pageurl = new moodle_url('/admin/plugins.php', $pageparams);
 
 $pluginman = core_plugin_manager::instance();
 
+$PAGE->set_primary_active_tab('siteadminnode');
+
 if ($uninstall) {
 
     if (!$confirmed) {