Implement first AC in 'User completes assignment'

Implement 'User sees a done button on the assignment' AC.
[51486139]
Implement 'User sees that lesson is completed'

Implement 'User sees that lesson is completed' AC. Add lesson
association to Coursework model. Nest assignments inside lessons for
show action.
[#51486139]
Merge branch 'master' into user_completes_assignment
Implement 'User completes assignment'

Complete 'User does not see a 'Done' button if assignment is already
completed.
[Finishes #51486139]