FIX: Blank dialog when auto-join selected in channel creation modal (#20405)

Fixes issue introduced in 7ef482a2920d32412b5c4d028dd45fda30670beb
where the correct warning message was not shown when enabling auto-join
for public categories when creating a channel. Adds more system specs
as well to avoid regressions.

spec/system/page_objects/components/dialog.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Components
+    class Dialog < PageObjects::Components::Base
+      def has_content?(content)
+        find(".dialog-container").has_content?(content)
+      end
+
+      def click_yes
+        find(".dialog-footer .btn-primary").click
+      end
+
+      def click_no
+        find(".dialog-footer .btn-default").click
+      end
+    end
+  end
+end