[change] Make menu item registration idempotent

Describe the bug
Initializing the app multiple times gives error.
Although this is not a main usecase, it can happen when running tests that override settings.

I noticed it when I tried to override installed apps to test migrations between two databases (created with slightly different settings).

Steps To Reproduce
Add a test which is overriding the INSTALLED_APPS setting.

For instance:

from django.test import TestCase
from django.test.utils import modify_settings

class TestDB(TestCase):

    @modify_settings(INSTALLED_APPS={'remove': 'django_filters'})
    def fake_test(self):
        pass

Expected behavior

The app should be able to be initialize multiple times, so that test example should simply pass.

Screenshots

$ ./manage.py test testapp.tests.test_db.TestDB.fake_test
Found 1 test(s).
Creating test database for alias 'default'...
System check identified no issues (0 silenced).
E
======================================================================
ERROR: fake_test (testapp.tests.test_db.TestDB.fake_test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/alexv/Projects/openwisp-users/.direnv/python-3.12/lib/python3.12/site-packages/django/test/utils.py", line 458, in inner
    with self as context:
         ^^^^
  File "/home/alexv/Projects/openwisp-users/.direnv/python-3.12/lib/python3.12/site-packages/django/test/utils.py", line 423, in __enter__
    return self.enable()
           ^^^^^^^^^^^^^
  File "/home/alexv/Projects/openwisp-users/.direnv/python-3.12/lib/python3.12/site-packages/django/test/utils.py", line 605, in enable
    super().enable()
  File "/home/alexv/Projects/openwisp-users/.direnv/python-3.12/lib/python3.12/site-packages/django/test/utils.py", line 492, in enable
    apps.set_installed_apps(self.options["INSTALLED_APPS"])
  File "/home/alexv/Projects/openwisp-users/.direnv/python-3.12/lib/python3.12/site-packages/django/apps/registry.py", line 362, in set_installed_apps
    self.populate(installed)
  File "/home/alexv/Projects/openwisp-users/.direnv/python-3.12/lib/python3.12/site-packages/django/apps/registry.py", line 124, in populate
    app_config.ready()
  File "/home/alexv/Projects/openwisp-users/openwisp_users/apps.py", line 27, in ready
    self.register_menu_group()
  File "/home/alexv/Projects/openwisp-users/openwisp_users/apps.py", line 66, in register_menu_group
    register_menu_group(
  File "/home/alexv/Projects/openwisp-users/.direnv/python-3.12/lib/python3.12/site-packages/openwisp_utils/admin_theme/menu.py", line 192, in register_menu_group
    raise ImproperlyConfigured(
django.core.exceptions.ImproperlyConfigured: A group/link with config {'label': 'Users & Organizations', 'items': {1: {'label': 'Users', 'model': 'openwisp_users.User', 'name': 'changelist', 'icon': 'user'}, 2: {'label': 'Organizations', 'model': 'openwisp_users.Organization', 'name': 'changelist', 'icon': 'ow-org'}, 3: {'label': 'Groups & Permissions', 'model': 'openwisp_users.Group', 'name': 'changelist', 'icon': 'ow-permission'}, 4: {'label': 'Organization Owners', 'model': 'openwisp_users.OrganizationOwner', 'name': 'changelist', 'icon': 'ow-org-owner'}, 5: {'label': 'Organization Users', 'model': 'openwisp_users.OrganizationUser', 'name': 'changelist', 'icon': 'ow-org-user'}}, 'icon': 'ow-user-and-org'} is being registered at position "40",                but another group named "Users & Organizations" is already registered at the same position.

----------------------------------------------------------------------
Ran 1 test in 0.003s

System Informatioon:

• OS:

$ cat /etc/issue
Pop!_OS 22.04 LTS

• Python Version: Python 3.12.7

$ pip list | grep 'openwisp\|django'
django-admin-autocomplete-filter 0.7.1
django-allauth                   65.7.0
django-compress-staticfiles      1.0.1b0
django-extensions                4.1
django-filter                    23.5
django-model-utils               5.0.0
django-organizations             2.5.0
django-phonenumber-field         8.1.0
django-redis                     5.4.0
django-reversion                 5.1.0
django-sesame                    3.2.3
djangorestframework              3.15.2
openwisp-users                   1.2.0a0     /home/alexv/Projects/openwisp-users
openwisp-utils                   1.2a0

Start in openwisp_users/apps.py, especially ready() and register_menu_group(), then run the INSTALLED_APPS override reproducer from the issue. Confirm that initializing the app multiple times no longer raises the duplicate menu-group error and that the test completes successfully.