replace Risk with ThreatLevel [skip ci]
    - Event.risk has been replaced by Event.threat_level_id.
      all functionality remains the same and users should not see
      any difference.
      ENUM() used for Event.risk is vendor specific and requires
      too many hacks to play nicely with bake.
    - Added default schema file, SQL dumps should be avoided since
      they make updating/upgrading a pain.
    - Removed old unused schemas