Parse and store contact list

[mplorentz]

Nostr clients publish the list of accounts they are following in a kind: 3 event. The format of these events is documented in NIP-02. We should add a Core Data model (call it Follow?) to hold this information and extend EventProcessor to parse and save them. This will be a good place to use TDD since the RelayService doesn't prioritize downloading kind 3 events necessarily, so it might be hard to test in production.

This is a pre-requisite to following other profiles.