Audit trail for device changes.

This issue has been migrated from [#5145](https://github.com/matrix-org/synapse/issues/5145).

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We need an API which exposes what devices get created and removed on your account.

Synapse should also mail & push you when devices get created (i.e. people log in to your account), to track abuse.

Ideally this needs to somehow be resilient to a malicious homeserver admin (otherwise an admin might add malicious devices to your account and remove them again without you noticing). The way to do that is probably to expose device additions more clearly in the apps, rather than waiting to see a KS req from an unexpected device or similar.