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.