Navigation customisation within plugins should use 'navigation' namespace (backport of MDL-74733)

Test instructions copied from MDL-74733 : On a test course, you will need one of each of the following activity types: assign, feedback, glossary, label, quiz. These can be created with all default options; you don't need any content. Go into each activity type (for the label it doesn't have its own page, but you can go to the settings page for it). CHECK there are no errors or warnings. Go into the 'Feedback' activity, using a large window so that it has the full space available, and look at the secondary navigation. CHECK the 'Responses' link is one of the top-level options (this is testing that the custom navigation works; without it, this link ends up in the More menu). You will also need an HTML block (if you don't already have one, go to blocks, add a block, and choose 'Text'). Click the 'Configure' icon in the block header. CHECK the settings form appears with no errors or warnings. There is no secondary navigation on this page.