Reinstate template for groups/index.html

As per https://github.com/wagtail/wagtail/pull/10540/files#r1226830421 - while we are using generic/index.html unmodified here, giving it a dedicated template file allows people to override it if desired.

wagtail/users/templates/wagtailusers/groups/index.html

@@ -0,0 +1 @@
+{% extends "wagtailadmin/generic/index.html" %}

wagtail/users/tests/test_admin_views.py

@@ -1291,6 +1291,7 @@ class TestGroupIndexView(WagtailTestUtils, TestCase):
     def test_simple(self):
         response = self.get()
         self.assertEqual(response.status_code, 200)
+        self.assertTemplateUsed(response, "wagtailusers/groups/index.html")
         self.assertTemplateUsed(response, "wagtailadmin/generic/index.html")
         # response should contain page furniture, including the "Add a group" button
         self.assertContains(response, "Add a group")

wagtail/users/views/groups.py

@@ -53,6 +53,7 @@ class PermissionPanelFormsMixin:
 
 
 class IndexView(generic.IndexView):
+    template_name = "wagtailusers/groups/index.html"
     results_template_name = "wagtailusers/groups/results.html"
     page_title = _("Groups")
     add_item_label = _("Add a group")