Migrate incident tracking to ooni/backend

[hellais]

In the last weekly backend call we agreed to:

• Close all incident tickets in ooni/sysadmin and if necessary create next steps issues in ooni/backend
• Add documentation on what to do for post mortem of incidents mentioning that older incidents where in ooni/sysadmin, but new ones should be filed in ooni/backend
• Move the tracking issues in ooni/sysadmin into ooni/backend following the new guidelines