Migration of Django admin to Unfold

[giohappy]

The plans for GeoNode 5 include a new central role for the Django Admin section. Some GeoNode functionality currently implemented with legacy views and templates will be moved here, and new planned admin features will probably be located under the admin area.

Given the new centrality of this section, we want to give it a restyling and rename it to "GeoNode Back Office".
The proposed theme is unfold, which provides a nice-looking UI and pretty useful features for the layout of the admin tools.

The branch with the unfold admin is https://github.com/GeoNode/geonode/tree/unfold_admin