Bug fixes (v4)

- Improve intelligence for next question type based on the new methodology of limiting Consecutive answers (Days next due). - Integrated new quality algorithm to establish users' capability when serving smart question types. For example: If user has 3/3 questions correct, we should not serve them multiple choice questions anymore (too easy) - Automatically move to typing mode, or audio, as per their settings. If they have 1/1 (first answer multiple), then typing mode is more likely to appear next. - Fix courses due page - show up to 50 instead of 10, to allow users with many concurrent courses being learnt to see all of them. - Implement tests to 'guarantee' questions due count matches up with the courses they have 'hidden' from dashboard. There still may be slight discrepancies. The calculation for each user is very database-heavy, therefore it is cached for 15 minutes.