Auto-Delete Closed Polls

[danth]

When a poll is closed, either by a timer or manually with pm!close, there should be an option to automatically delete it after a certain number of days (similar to running pm!delete).

This prevents the label namespace from getting clogged up, while still allowing adequate time to export the poll if necessary. The number of days to keep polls would be configurable by server administrators, and it is disabled by default.