message: UsersStack: add xs (16px) size option
diff: background: var(--content_placeholder_icon);
  }
  +.UsersStack--size-xs .UsersStack__photo {
  + width: 16px;
  + height: 16px;
  +}
  +
  .UsersStack--size-s .UsersStack__photo {
  width: 24px;
  height: 24px;
  margin-left: -2px;
  }
  +.UsersStack--size-xs:not(.UsersStack--simple) .UsersStack__photo + .UsersStack__photo {
  + margin-left: -1px;
  +}
  +
  .UsersStack--size-s:not(.UsersStack--simple) .UsersStack__photo + .UsersStack__photo {
  clip-path: url(#users_stack_mask_24_left);
  }
  