Implement expiration delay for workspaces

Part of #542 (closed). See: https://salsa.debian.org/freexian-team/debusine/-/blob/devel/docs/reference/devel-blueprints/experiment-workspaces.rst

Adding these fields to Workspace:

• created_at = models.DateTimeField(auto_now_add=True) (!1612 (merged))
• expiration_delay = models.DurationField(blank=True, null=True) (!1612 (merged))
• implement workspace expiration in regular maintenance (!1612 (merged))
• show a workspace expiration in the UI, and allow workspace owners to change/extend it (!1625 (merged))

If feasible, workspaces should expire expiration_delay days after max(created_at, last_task_completion_time), instead of after created_at.