mail_new_activity: use Story method for determining if user hid the story
mail_new_activity: use Story method for determining if user hid the story

Fixes #141

nh-platform/barnacles,sboklund/munu.co,Denniskevin/lobsters,ABorgna/lobsters,ABorgna/lobsters,Bloc/lobsters,chaica/lobsters,sboklund/munu.co,Bloc/lobsters,nh-platform/barnacles,saitodisse/lobsters,sboklund/munu,Denniskevin/lobsters,mojacc/lobsters,Bloc/lobsters,GetStream/lobsters,saitodisse/lobsters,saitodisse/lobsters,sboklund/munu,GetStream/lobsters,sboklund/munu,Denniskevin/lobsters,GetStream/lobsters,Denniskevin/lobsters,saitodisse/lobsters,Bloc/lobsters,sboklund/munu.co,nh-platform/barnacles,ABorgna/lobsters,mojacc/lobsters,nh-platform/barnacles,sboklund/munu.co,derekprior/lobsters,sboklund/munu,chaica/lobsters,ABorgna/lobsters,derekprior/lobsters,chaica/lobsters,GetStream/lobsters,derekprior/lobsters,mojacc/lobsters,derekprior/lobsters,mojacc/lobsters,chaica/lobsters