Removes $PAGE->set_secondary_navigation(false) calls which certain pages
were using to prevent displaying secondary navigation menu in the system
context. This logic is now generally handled within the secondary
navigation class.
- Introduce core_course_category::get_nearest_editable_subcategory()
- This function will return the first creatable/manageable category
for current user
- With this new function, we can fix the issue that the users with
course management or creation permision at category level cannot see
the manage menu on My courses page
