Add a PII-correlation tool for moderators

[ArtOfCode-]

Add a tool which enables moderators to identify where there are PII correlations between users. This should be a user-specific mod tool (i.e. accessible via user mod tools rather than the community-wide mod tools menu). This should:

• allow moderators to select a specific other user against whom to correlate the current user's PII
• protect user PII by not revealing exact PII values
  • where it's necessary to display a value to moderators, this should be a hash of the stored value rather than the information itself
  • information may be broken down if this helps to identify connections (i.e. email address into handle and domain, IP address into octets) but each part should still be hashed
  • it may help to display only the first n characters of a hash for readability, but the full hash should be available and/or the UI should clearly display if two hashes are identical
• simply display a measure of correlation (as hashes for very similar values will be completely different)
  • an IP address where 3 of 4 octets are identical is a stronger correlation than just the first octet (consider also IPv6)
  • an email address on the same domain may mean nothing for gmail.com, but could mean everything for a personal domain; consider some way to measure how commonly used an email domain is
• [optional] separately from specific-user selection, may suggest other users whose PII has some correlation with the current user
  • this will require additional querying - consideration needed to optimisation; potentially gated behind an additional click rather than always requested