improved Proposal.proposal
improved Proposal.proposal

 * renamed the field to avoid nasty underscore
 * fixed code to avoid extra query

pinax/symposion,toulibre/symposion,pyconau2017/symposion,faulteh/symposion,pydata/symposion,TheOpenBastion/symposion,mbrochh/symposion,euroscipy/symposion,miurahr/symposion,TheOpenBastion/symposion,euroscipy/symposion,miurahr/symposion,pyconca/2013-web,python-spain/symposion,pyohio/symposion,mbrochh/symposion,pinax/symposion,faulteh/symposion,python-spain/symposion,pydata/symposion,pyconau2017/symposion,NelleV/pyconfr-test,toulibre/symposion,pyconca/2013-web,NelleV/pyconfr-test,pyohio/symposion