Configuration to limit the number of active devices a single user may have

This issue has been migrated from [#16064](https://github.com/matrix-org/synapse/issues/16064).

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**Description:**

Currently, the number of devices a single user can have is unbounded. This can start to cause performance concerns, especially when things involving inter-device messaging are concerned. It would be good to allow service providers the option to set a hard cap on the number of devices a user can have (prompting them to log out of older devices that may not be in use).

When a user hits their caps, I'd expect new /login attempts to be blocked.

I suspect this kind of thing will need a spec of some kind, at the very minimum a new error code to describe the the failure.

**EDIT:** Oh and if it wasn't clear, this is primarily for the non-federated use case. Obviously some person could join your room with several hundred devices and ruin things, but small steps etc.