Risk observatory pipeline

• add subcommands for risk observatory pipeline main entry point
  • revert_risk_prediction to run batch prediction of the revert risk model
  • highrisk_thresholds to compute optimal thresholds for classifying high risk revisions
  • aggregate_statistics to pre-aggregate statistics displayed by the risk observatory dashboard
• adds wikidiff pipeline as risk observatory pipeline is dependent on in
• migrates the revert risk ml dataset pipeline as it shares a number of steps with the risk observatory pipeline
• add/move unit tests for all pipelines