fix: Add (back) navigation extenders to advanced settings

[fsbraun]

Description

It seems that when restructuring the advanced settings form, navigation_extenders haven been dropped from the fields:

django-cms/cms/admin/forms.py

Lines 684 to 690 in 52e6f87

	fields = [
	'login_required',
	'reverse_id',
	'application_urls',
	'application_namespace',
	'application_configs',
	]

The functionality seems untouched. I have not found any reasoning in the release notes or commits removing the navigation_extenders. Since the commit was a rather large refactor, I can imagine that dropping the navigation extenders was not on purpose.

This PR adds back the navigation_extenders field.

Related resources

• Test have always stayed in the codebase: https://github.com/django-cms/django-cms/blob/develop-4/cms/tests/test_navextender.py
• #...

Checklist

• I have opened this pull request against develop-4
• I have added or modified the tests when changing logic -> n/a
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.