UX: Move group mentions notifications into the reply tab (#22562)

Why this change?

Group mention notifications are currently placed in the "Others" tab
of the user menu which is odd considering that mentioned notifications
are in the reply tab. This commit changes it such that group mention
notifications are displayed in the reply tab as well.

spec/system/page_objects/components/user_menu.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Components
+    class UserMenu < PageObjects::Components::Base
+      def open
+        find(".header-dropdown-toggle.current-user").click
+        has_css?(".user-menu")
+        self
+      end
+
+      def click_replies_notifications_tab
+        click_link("user-menu-button-replies")
+        has_css?("#quick-access-replies")
+        self
+      end
+
+      def has_group_mentioned_notification?(topic, user_that_mentioned_group, group_mentioned)
+        expect(find("#quick-access-replies .group-mentioned").text).to eq(
+          "#{user_that_mentioned_group.username} @#{group_mentioned.name} #{topic.title}",
+        )
+      end
+
+      def has_right_replies_button_count?(count)
+        expect(find("#user-menu-button-replies").text).to eq(count.to_s)
+      end
+    end
+  end
+end