FEATURE: Use direct_message_enabled_groups for DM access (#1209)

This commit introduces the direct_message_enabled_groups setting,
which is used to control which groups can send direct messages in
chat. Staff can always send direct messages.

If a user is not part of any direct_message_enabled_groups,
then they cannot create new DM channels, cannot send messages
in existing DM channels, and if they have no existing DM channels
they will not see the DM channel list in the sidebar.

assets/javascripts/discourse/components/channels-list.js

@@ -17,6 +17,27 @@ export default class ChannelsList extends Component {
   @reads("chat.directMessageChannels.[]") directMessageChannels;
   @empty("publicChannels") publicChannelsEmpty;
 
+  @computed("canCreateDirectMessageChannel")
+  get createDirectMessageChannelLabel() {
+    if (!this.canCreateDirectMessageChannel) {
+      return "chat.direct_messages.cannot_create";
+    }
+
+    return "chat.direct_messages.new";
+  }
+
+  @computed("canCreateDirectMessageChannel", "directMessageChannels")
+  get showDirectMessageChannels() {
+    return (
+      this.canCreateDirectMessageChannel ||
+      this.directMessageChannels?.length > 0
+    );
+  }
+
+  get canCreateDirectMessageChannel() {
+    return this.chat.userCanDirectMessage;
+  }
+
   @computed("directMessageChannels.@each.last_message_sent_at")
   get sortedDirectMessageChannels() {
     if (!this.directMessageChannels?.length) {