Support for groups of questions (e.g. exercises)

[nthiery]

Scenario:

• At any time, the user can create a group of questions, providing a label (typically Exercise 1).
• Upon creating a question, the user can assign it to a group of questions, through
  a dropdown menu. The assignment can be updated later on.
  Bonus: by default, the question is assigned to the latest group of questions that
  was created or assigned to.
• A default question label of the form Exercise 1.3 (for the third exercise assigned to the
  group) is proposed.
  Bonus: the question label is automatically updated.
• Upon computing scores, a cumulative score per group of question is calculated.

Open question: is there a use case for a recursive structure: groups of groups of questions?