Dice management

[JakobKallin]

We should be able to leave our physical dice in the bag if we want to, instead letting Fiascomputer roll and manage them for us!

One issue with doing this is the rigidity of strictly enforcing the dice rules: if the app actually prevents users from assigning invalid choices, we're making things a lot more tedious for groups using house rules. (Also, remember that the Fiasco rules themselves say that you can change elements if it makes the game better, even when it's not supported by your dice pool.) If we allow users to switch the dice rules on and off as needed, we need to decide what happens when they're switched on or off in the middle of a setup.