getSubtitle method
- RxUser? partner,
Returns string represented subtitle of this Chat.
If isGroup, then returns the members length, otherwise returns the
presence of the provided partner, if any.
Implementation
String? getSubtitle({RxUser? partner}) {
  switch (kind) {
    case ChatKind.dialog:
      return partner?.user.value.getStatus(partner.lastSeen.value);
    case ChatKind.group:
      return 'label_subtitle_participants'.l10nfmt({'count': membersCount});
    case ChatKind.monolog:
    case ChatKind.artemisUnknown:
      return null;
  }
}