Reporting on practice challenges

[sei-bstein]

Questions to answer:

• Total practice challenges completed
• Most popular practice challenges
• How many users are interacting with the Practice module?
• Number of practice challenges per user, average practice challenges completed per user, graphs (?)

[sei-bstein]

Addressed most of this in the new Practice report. The only item listed here that we didn't catch is "Average practice challenges completed per user", which i interpret to be a global stat rather than a user-level one. We do report how many practice challenges each user has completed, but no global average. I'm going to close this issue as a duplicate and add this item to #126.