Fraud rings from shared devices and cards

A transaction log records which user used which device and which card. Investigate one target user in three steps:

1. Return the users who used the same device as the target (excluding the target), sorted.
2. A fraud ring is everyone connected to the target through any chain of shared devices or cards. Return the size of the target's ring.
3. Each log line also carries a risk score. Block the target's ring when the average risk of its users is above 75, where users with score 0 are trusted: they still connect the ring but are left out of the average.