Add User.find_by_user_name classmethod
Add User.find_by_user_name classmethod

This is expected in lib.importexport._Transform._get_by_key.

DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,phihag/adhocracy,alkadis/vcv,phihag/adhocracy,alkadis/vcv,phihag/adhocracy,alkadis/vcv,alkadis/vcv,phihag/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,alkadis/vcv