Check that transaction is balanced on save
Check that transaction is balanced on save

This adds a sqlalchemy.event listener to listen about inserts
or updates of transactions and abort them if its not balanced

agdsn/pycroft,lukasjuhrich/pycroft,agdsn/pycroft,agdsn/pycroft,lukasjuhrich/pycroft,agdsn/pycroft,agdsn/pycroft,lukasjuhrich/pycroft,lukasjuhrich/pycroft