[FOLLOW-UP] Additional message processing from chat-relay

[Antreesy]

Missing from chat-relay:

• fix(chat-relay): implement fallback polling #16687
  • Once per 2 minutes
  • Fetch back from the last known? forward from the last given by server?
• fix(chat-relay): localize system messages #16406
  • Check for guests, email guests, federated users
  • API: fix(Listener.php): provide a silent call flag through a chat-relay #16886

Two bonuses to process:

• Poll sys messages now include additional information feat(poll): improve poll card and dialog UI #16137
• Thread object is included in thread-created sys message

Originally posted by @DorraJaouad in #16406 (comment)