Use valid_on instead of posted_at for transactions
Use valid_on instead of posted_at for transactions

Closes #233

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