fix: Grid compatibility for django CMS v4 plugin architecture

[Aiky30]

Fixes the following error thrown when trying to create a plugin using the bootstrap grid:

AttributeError at /admin/cms/placeholder/add-plugin/
'Bootstrap4GridRow' object has no attribute 'numchild'

[crydotsnake]

TODO:
Error when adding the Link/Button plugin too the page

Error Log:

Environment:


Request Method: GET
Request URL: http://127.0.0.1:8000/admin/cms/placeholder/add-plugin/?placeholder_id=3&plugin_type=Bootstrap4LinkPlugin&cms_path=/admin/cms/placeholder/object/5/edit/2/&plugin_language=en&plugin_position=9

Django Version: 2.2
Python Version: 3.8.8
Installed Applications:
['quickstart',
 'djangocms_admin_style',
 'django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.sites',
 'cms',
 'menus',
 'treebeard',
 'sekizai',
 'filer',
 'easy_thumbnails',
 'djangocms_versioning',
 'djangocms_alias',
 'djangocms_text_ckeditor',
 'djangocms_file',
 'djangocms_icon',
 'djangocms_link',
 'djangocms_url_manager',
 'djangocms_picture',
 'djangocms_style',
 'djangocms_googlemap',
 'djangocms_video',
 'djangocms_bootstrap4',
 'djangocms_bootstrap4.contrib.bootstrap4_alerts',
 'djangocms_bootstrap4.contrib.bootstrap4_badge',
 'djangocms_bootstrap4.contrib.bootstrap4_card',
 'djangocms_bootstrap4.contrib.bootstrap4_carousel',
 'djangocms_bootstrap4.contrib.bootstrap4_collapse',
 'djangocms_bootstrap4.contrib.bootstrap4_content',
 'djangocms_bootstrap4.contrib.bootstrap4_grid',
 'djangocms_bootstrap4.contrib.bootstrap4_jumbotron',
 'djangocms_bootstrap4.contrib.bootstrap4_link',
 'djangocms_bootstrap4.contrib.bootstrap4_listgroup',
 'djangocms_bootstrap4.contrib.bootstrap4_media',
 'djangocms_bootstrap4.contrib.bootstrap4_picture',
 'djangocms_bootstrap4.contrib.bootstrap4_tabs',
 'djangocms_bootstrap4.contrib.bootstrap4_utilities']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
 'whitenoise.middleware.WhiteNoiseMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware',
 'django.middleware.locale.LocaleMiddleware',
 'cms.middleware.user.CurrentUserMiddleware',
 'cms.middleware.page.CurrentPageMiddleware',
 'cms.middleware.toolbar.ToolbarMiddleware',
 'cms.middleware.language.LanguageCookieMiddleware']



Traceback:

File "/usr/local/lib/python3.8/site-packages/django/core/handlers/exception.py" in inner
  34.             response = get_response(request)

File "/usr/local/lib/python3.8/site-packages/django/core/handlers/base.py" in _get_response
  115.                 response = self.process_exception_by_middleware(e, request)

File "/usr/local/lib/python3.8/site-packages/django/core/handlers/base.py" in _get_response
  113.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/usr/local/lib/python3.8/site-packages/django/utils/decorators.py" in _wrapped_view
  142.                     response = view_func(request, *args, **kwargs)

File "/usr/local/lib/python3.8/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
  44.         response = view_func(request, *args, **kwargs)

File "/usr/local/lib/python3.8/site-packages/django/contrib/admin/sites.py" in inner
  223.             return view(request, *args, **kwargs)

File "/usr/local/lib/python3.8/site-packages/django/views/decorators/clickjacking.py" in wrapped_view
  33.         resp = view_func(*args, **kwargs)

File "/usr/local/lib/python3.8/site-packages/cms/admin/placeholderadmin.py" in add_plugin
  375.         response = plugin_instance.add_view(request)

File "/usr/local/lib/python3.8/site-packages/django/contrib/admin/options.py" in add_view
  1634.         return self.changeform_view(request, None, form_url, extra_context)

File "/usr/local/lib/python3.8/site-packages/django/utils/decorators.py" in _wrapper
  45.         return bound_method(*args, **kwargs)

File "/usr/local/lib/python3.8/site-packages/django/utils/decorators.py" in _wrapped_view
  142.                     response = view_func(request, *args, **kwargs)

File "/usr/local/lib/python3.8/site-packages/django/contrib/admin/options.py" in changeform_view
  1522.             return self._changeform_view(request, object_id, form_url, extra_context)

File "/usr/local/lib/python3.8/site-packages/django/contrib/admin/options.py" in _changeform_view
  1575.                 form = ModelForm(initial=initial)

File "/usr/local/lib/python3.8/site-packages/djangocms_link/cms_plugins.py" in __init__
  67.                 self.for_site(site)

File "/usr/local/lib/python3.8/site-packages/djangocms_link/forms.py" in for_site
  21.         self.fields['internal_link'].queryset = Page.objects.drafts().on_site(site)

Exception Type: AttributeError at /admin/cms/placeholder/add-plugin/
Exception Value: 'PageManager' object has no attribute 'drafts'

[Aiky30]

@crydotsnake Your issue raised here has been moved to a separate issue to help unblock the changes fixed in this PR.

#139