Show completion requirements in the frontend

[mipyykko]

A new field required_for_completion in UserSummary, which provides a list objects containing information for requirements to complete the course.

Now:

• not enough points, if course has points_needed set
• not enough exercise completions, if course has exercise_completions_needed set
• required actions from quiz providers such like peer review requirements, which are just the exercise_completion_required_action for this course/user combination

Query and visualize these in the frontend.