add joined nostr date for profiles

[alltheseas]

Example profile: https://njump.me/npub17jlav3akkp4eps6k2ke9uq2pcg6erdfj4kae9r4tsggttjamcnesyw4k7x

mockup

[dtonon]

Having this date would means knowing the first signed event. We probably can't query real-time relays on this, since they purge old events, so we should cache it somewhere.