True question-based learning progress

Changes the learning progress calculation for the question-based option.

Todos until this work is complete:

• Count inactive questions if they have answers. Right now, inactive questions are filtered out completely. For example, teachers might enable questions for a limited amount of time, but still want to see the learning progress after they deactivate those questions.
• Check caching implementation. Requests for "my learning progress" take more than 2s!