Add required functions for navigation_node.

Within this sub-task we need to add functionality to fetch navigation_node's to populate the navigation view by calling the settings_navigation functions that generate either course, module or user context nodes. It may be likely that we emulate a lot of the initialise() to populate this new navigation.

Acceptance criteria

• Contents of secondary navigation changes depending on the users context
• Confirm that menu items added by core modules appear within secondary navigation
• Third party modules can add navigation items via *_extend_settings_navigation