Show "Mark Complete" button for a Lesson when Quiz and/or Assignment have been completed

[brianhogg]

Is your feature request related to a problem? Please describe.
Current a student can click "Mark Incomplete" to mark the lesson as incomplete, on a lesson with a quiz and/or assignment. However they cannot undo that change without retaking the quiz.

Describe the solution you'd like
Show a "Mark Complete" button if the lesson is complete, and they are permitted to mark the lesson as completed (ie. the quiz has already been taken).