FIX: Allow slash commands to set rules in private groups

The Slack 'slash commands' API does not provide the user-friendly group name, so we must use the group ID. We could use the API to obtain the group name, but it would require adding additional scopes to the app.

lib/discourse_chat/provider/slack/slack_command_controller.rb

@@ -28,6 +28,8 @@ module DiscourseChat::Provider::SlackProvider
         case params[:channel_name]
         when 'directmessage'
           "@#{params[:user_name]}"
+        when 'privategroup'
+          params[:channel_id]
         else
           "##{params[:channel_name]}"
         end

lib/discourse_chat/provider/slack/slack_provider.rb

@@ -6,7 +6,7 @@ module DiscourseChat::Provider::SlackProvider
   PROVIDER_ENABLED_SETTING = :chat_integration_slack_enabled
 
   CHANNEL_PARAMETERS = [
-                        { key: "identifier", regex: '^[@#]\S*$', unique: true }
+                        { key: "identifier", regex: '^[@#]?\S*$', unique: true }
                        ]
 
   def self.excerpt(post, max_length = SiteSetting.chat_integration_slack_excerpt_length)