cubicweb #1631319 ask user before trying to install plpythonu / plpgsql [validation pending]

Currently, with a postgresql backend, cubicweb creates both plpythonu and plpgsql languages, except on win32 where plpythonu is simply not created.

A (sligthly) better solution would be for cubicweb to ask if the language creation is needed or not. This would avoid crashes on systems where plpythonu is not installed but not required.

done in3.13.0
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