Review background selector workflow

[allyoucanmap]

The new background selector allows user to manage both background and 3D terrain. The background selector exposes functionalities to add, edit and remove background that cannot currently used in GeoNode.

We should allows user to modify the backgrounds inside maps as now it's possible in MapStore.

Requirements:

• new maps should still use the background list provided by the backend settings.py
• while editing a user can decide to modify background by add, editing or remove background. This changes needs to be saved and persisted in the map configuration
• options to edit, add and remove a background should not be available to user without editing permissions
• options to edit, add and remove a background should not be available inside the dataset viewers (all pages)