Don't show locked items in inventory.

[miguelmarco]

When you start playing a game that eventually unlocks a lot of theorems/definitions/tactics, you see them in the inventory from the beginning (in a different color and with a lock sign, but they are visible). This can be overwhelming for the people that play the game from the beginning.

I propose to only show the items that are unlocked. This would also have the advantage of making the progress in the game more rewarding for the player, since (s)he would see the inventory grow as (s)he progresses in the game.

[miguelmarco]

I created #266, that hides them with css.