Management command to clear old incident reports

[rspeed]

Named clearincidentreports. Similar to clearsessions.

• Option to keep reports newer than a certain date/time period
  • ./manage.py clearincidentreports --before 2017-06-01
  • ./manage.py clearincidentreports --before 1 week
• Ability for it to be run automatically through a cronjob, etc.
• Add the ability to flag incident reports to keep.
  • Using a custom Admin Action.
  • From the individual report view. Might conflict with Add a custom Admin view for reports #19.