ziafazal/api-time-series-invalid-data: fix negative values in users_not_started

@mattdrayer calculating time series data from `StudentProgress` instead of `StudentProgressHistory` to avoid negative values for `users_not_started` metrics.  @dino-cikatic I'm making `users_enrolled` metrics same as other so it mean it would add up number of users enrolled earlier than start date as does the other metrics. Fixed some broken tests.
Does StudentProgress also contain the history? Otherwise how can we get all the time series data?