Using existing PGP signatures for tracking

There are several users I want to track on Keybase whose PGP keys I've already signed. Before tracking them, I'm going to manually compare their fingerprints against the already-signed fingerprints on my local keychain. It seems like it would be nice to do that more automatically.

No files, tests, or entry points are named. Start by locating the Keybase client code for tracking users and local keychain or PGP fingerprint handling, then determine how existing signatures should affect tracking. Done means the client can use matching locally signed fingerprints instead of requiring the comparison to be performed manually.

Written by the indexing model from the issue text.