Bam, add exercise log export option
Merge pull request #2379 from learningequality/exerciselogcsv

Bam, add exercise log export option
Revert "use logging rather than writing to stdout directly" for collectstatic

This reverts commit 239fa61bd5bf577503be9b90830dd466194cfb55.
Added Test facility for A/B testing.
Merge branch 'nalanda-rct3' of github.com:learningequality/ka-lite into nalanda-rct3
Uses .get instead of keying directly into UNIT_EXERCISES dict.
Prevents exercise api endpoint from breaking.