Show lastActivity on online races

[codicocodes]

Backend:

• Save last_activity on race state
• Update last_activity on the following events:
  • play
  • refresh
  • key_stroke
• Include lastActivity on public race

Frontend:

• display lastActivity using humanizeRelative in the public races modal together with the specific public race

It would say "las active 7 minutes ago" e.g.