Enhancing course progression and navigation abilities

[westnz]

Is your feature request related to a problem? Please describe

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I'm always frustrated that there are no 'previous' and 'next' buttons on a lesson within a course. When I want to return to a previous or different lesson, I must select the relevant lesson using the sidebar.

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A 'previous' and 'next' button should be added to the bottom and top of a lesson to navigate quickly between lessons within a course.

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