Cleanup and improve admin settings navigation

[malteschmitz]

This pull requests performs some refactoring to clean the code and optimise the UI of the admin settings navigation tree:

• require_once("$CFG->dirroot/mod/assign/adminlib.php") is not needed. We do not need a library from mod_assign.
• I tried to make the names of the admin tree nodes more consistent and tried to following the implicit Moodle conventions:
  • tool_monitoring is the category
    • tool_monitoring_overview is the external page with the overview table
    • tool_monitoring_exporters is the category for the exporters
• The admin external page now uses admin_externalpage_setup('tool_monitoring_overview') and $PAGE->set_secondary_active_tab('modules') to correctly setup this page as an admin external page. As a result the title, header, breadcrumb navigation and the active indicator in the secondary navigation are now shown correctly. See below for a before and after screenshot.

Before:

After:

Our admin category for comparison:

Our metrics overview admin external page now has the same look and feel as the regular admin category page.