Include geonode-importer inside the core

[mattiagiupponi]

Is your feature request related to a problem? Please describe.
Since the geonode-importer is stable and is a core component in GeoNode, is better to have it along with the core.
This will allow us to:

• Remove everything that is not required in the old upload flow
• Remove unused functions/models
• Include a core component in the vanilla installation without adding extra dependencies in the master

Describe the solution you'd like
To avoid losing the history of the upload app, the importer will be moved inside the upload by adjusting the migrations. This will let us clean up some dead code

Describe alternatives you've considered
None

Additional context
None