Add to flow analytics: Which problems are the most time-consuming?

- For each answer-provided page visit, search for a preceding no-answer page visit. Use that as "time taken".
- For this time taken figure, provide summary statistics: min, max, mean, standard deviation.
- Include a toggle: Session from everyone/only "included in statistics". (since I would like to be able to see where TAs spend time during test runs)

Do this via the database, not in application code, if possible.