Download profile info for all displayed authors

[mplorentz]

Nostr identities at their core are public private keypairs. Profile data like usernames and profile photos are published in kind: 0 events as described in NIP-01. We parse this data and store it in the Author model when we see these events, but we are only fetching the most recent events from relays and often the kind: 0 events are not included.

We need to notice when we are displaying a post from a user we don't have profile data from. When this happens we should ask the RelayService to fetch it, store it in Core Data, and update our views with the new info.