Add some way to automatically expire stale devices

[turt2live]

This could potentially be controlled by the server alone, however it may be useful for users to decide how long 'stale' is.

[richvdh]

related matrix-org/synapse#8263

[uhoreg]

Something similar could be done mostly client-side right now, where a client could periodically check your devices and prompt you to clean them up if there are any that haven't been active for a long time. However, if this was done, we'd probably want to have some synchronization between clients so that your clients don't all yell at you. This would also not help with the bot-user case.

[notramo]

Often private browsing mode causes new devices (user doesn't log out before closing window).
Private mode can be detected. If it's detected, then Element Web should set a shorter inactivation time for that login, e.g. 1 week (if it is not seen for 1 week). Also, it should notify the user to always log out before closing private browsing window.

[t3chguy]

A browser being amnesiac can't often be detected. Private mode is just one common amnesiac environment.