Why this change?
When the site setting for chat_max_direct_message_users is set to 1, it is expected that users can have a 1:1 chat with other users. However, since the current user is counting as 1 user it makes starting a new chat impossible.
This change hands this validation off to DirectMessageChannel::MaxUsersExcessPolicy which handles the count correctly by filtering out the current user.
23fc0fb078