Ability to automatically kick out idle devices

This issue has been migrated from [#10813](https://github.com/matrix-org/synapse/issues/10813).

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Some users may want to kick out devices which haven't been used in N days and force the user to log back in, ensuring that if the device is compromised the attacker won't be able to read more than N days of history from the point of compromise.

See https://news.ycombinator.com/item?id=28513878