Allow the metadata editing when uploaded metadata is preserved

[giohappy]

The whole concept of "preserving uploaded metadata" is confusing as per https://github.com/GeoNode/geonode/issues/11448 and current behaviour: when "Metadata uploaded preserve" is selected, an error is issued when the "update" button is pressed (or when it is automatically triggered, see https://github.com/GeoNode/geonode/issues/11448).

The proposal is to clearly state that the preservation is only related to the uploaded XML document; this means that:

the metadata editing should still be possibile, since in the metadata editor there are some fields that can not be set using the values from the uploaded document when saving data from the metadata form, the new values should only be stored in the DB, but the XML document should not be regenerated. This could cause a misaligment between the DB fields (which are used in the CSW filtering) and the returned XML, but this change is up to the editor user, which should be notified of the "preserve" flag when saving metadata -- at the moment an error is always returned (and it's badly handled, see https://github.com/GeoNode/geonode/issues/11448).