[Backport 3.2.x][Fixes #7764] Implement GeoApp for Dashboards

[mattiagiupponi]

The first draft of dashboards integration into GeoNode

• GEONODE_APPS_INSTALLED required to dynamically populate the list of the available apps, use get_subclasses_by_model that will get the list of submodels for a given model

Checklist

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[giohappy]

@mattiagiupponi please reopen it for 3.3.x as soon as the new branch is created.