Store device ID alongside key backup, so we know who is uploading bad key backups.

This issue was created by the Crypto team because we believe it will help us to identify the source of broken key backups. We expect to work on it and hope the Synapse team will assist us in deciding what exactly to do, and helping us with review and merge assuming we can agree on a sensible approach.

We'd like to store in the Synapse DB which device uploaded a key backup, so we know which clients are uploading bad key backups.

Recently the Crypto team has identified broken key backups as a common source of "unable to decrypt" errors (UTDs). Some clients appear to be uploading invalid data, and other clients have sometimes been more strict than the spec in validating key backups.

Both of these problems result in UTDs, and it can be difficult when debugging to identify which client uploaded the key.

**Outcome of this task**

- Synapse populates its DB with the uploading device ID alongside each key backup, so that someone with access to the DB can identify the device linked to a rageshake