IMPR: fork fluxbb migration script for postgresql installs (PR #13031)

This was made to migrate an old postgres/fluxxbb setup: the existing fluxbb.rb migration script can only deal with a mysql db.


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I worry about a full fork of the import script just to switch the data source, stuff will drift.

Also sadly we can not review this at all without a CLA.

Strange, I have signed the CLA before I submitted the PR? I understand that a single script would be much better, I’m leaving this as it is for now, it might be useful for a future contribution.