prevent off-class items and free items being lost on death

Changes death so that you can lose only items that belong to your current class and items that belong to no class. See the [Trello death card](https://trello.com/c/k2MWBcxl/204-death-mechanic-v2) and https://github.com/HabitRPG/habitrpg/issues/2551

Also prevents free items being lost. Losing free items gives an unfair advantage to subscribers ("pay to win") because they have many more such items (Mystery Items) and so would have a greater chance of losing a free one. 

@lefnire or @paglias or anyone: I would love to know what's happening here with 'toObject' https://github.com/Alys/habitrpg-shared/blob/83b1804a555d11cc97634a7aa53da388311a2ad6/script/index.coffee#L401
It doesn't seem to be doing any damage, but ... what the heck??