[Fixes #12368] Introduce geonode-importer in geonode-core

[mattiagiupponi]

This PR will introduce the geonode-importer as a core module in geonode.
The PR includes also a cleanup of the no longer used models

Checklist

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[giohappy]

@mattiagiupponi some tests are failing. They don't seem related to this PR but let's give a look and fix them.

[ridoo]

This is an extensive PR with a lot of changes. It's good to see a lot of deprecated code disappearing and being replaced by the new importer approach. I never had a chance to really look at the old code, but some parts seemed a bit brittle to me, anyway. So good work.

So I just took a quick look and made a few mini comments. Anyway, feel free to continue as you wish.

[mattiagiupponi]

This is an extensive PR with a lot of changes. It's good to see a lot of deprecated code disappearing and being replaced by the new importer approach.

We know that the PR is huge, but it was the right moment to get rid of dead code and deprecated one.

I never had a chance to really look at the old code, but some parts seemed a bit brittle to me, anyway. So good work.

Lucky you! 😆

So I just took a quick look and made a few mini comments. Anyway, feel free to continue as you wish.

Comments are always welcome, feel free to add them