Add `closed_at` time for issues CSV export
We need to start analyzing average time to resolve of ~bug in each prioritization.
Exported sheet: https://docs.google.com/spreadsheets/d/105U1kg2nXutAbTzet2mCILO2zSx-ZFoEfXON-SmaIpA/edit#gid=14041422
We have created_at
updated_at
but no closed_at
The request here is to add the attribute so that data can be analyzed from CSV to start.