WIP Compact admin form UI with widgets attributes

[haricot]

same layout as related #365
This uses a widgets attributes for nested fields organized with specific helpers.

Be enabled with the environment variable: COMPACT_FORM=True (I tried with app_settings, but it doesn't work in CascadeUtilitiesMixin due to translation)

The widget cascade selector icon is associated with HTML / CSS / SVG and the fieldsets are defined according to the nested widget attribute group.

And this use a pull request #367 which also gives the possibility of filtering the fields for CascadeUtilitiesMixin.

git clone https://github.com/haricot/djangocms-cascade -b widget_attrs_group_nested
cd djangocms-cascade/examples/
python3.6 -m venv .venv
poetry shell
poetry install
npm install
./manage.py migrate
./manage.py createsuperuser

COMPACT_FORM=True ./manage.py runserver