FIX: Broken group messages inboxes when group name is mixed case (#22183)

This is a follow up to 1cbc65ba7949b0763c13347339b3064826023c07 where
visiting a group with a mixed case name would result in an error.

spec/system/page_objects/pages/user_private_messages.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Pages
+    class UserPrivateMessages < PageObjects::Pages::Base
+      def visit(user)
+        page.visit "/u/#{user.username}/messages"
+        self
+      end
+
+      def visit_group_inbox(user, group)
+        page.visit "/u/#{user.username}/messages/group/#{group.name}"
+        self
+      end
+
+      def has_right_inbox_dropdown_value?(value)
+        has_css?(".user-nav-messages-dropdown .combo-box-header[data-name='#{value}']")
+      end
+    end
+  end
+end