Provide option to delete (rather than archive) items at end of retro

[davidje13]

Action items should always be archived, but it should be possible to delete the retro items.

By default, the popup should show 3 options: Archive, Delete, Cancel. There should also be an option in the retro settings to always-archive or always-delete (with each removing the other button in the archive popup).

Alternatively, this could be a tick box in the archive popup; "erase retro items" (unticked by default but configurable as a setting).