[Fixes #7089] Delete existing table on restore command

[mattiagiupponi]

In order to introduce the new feature #7089, now by default, the restore management command will drop the existing table before restoring them.
If the existing tables/resources need to be preserved, the following argument is required:
--soft-reset -> will preserve tables and GeoServer resources

The main code is in utils.py.

I also change this line:

        pg_rstcmd += " -c" if preserve_tables else ""

In order to remove some warnings returned by pg_restore

This strategy is more offensive than conservative, but we can discuss it

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[mattiagiupponi]

Documentation update GeoNode/documentation#86