Implement true custom roll feature

We do have a Custom roll button, but it’s not a true custom roll. The Roll Custom screen limits us in dice mods to ability bonuses for the current character and a max bonus of +6. If you wanted to roll something like 1d20+15, there’s no good way to do that. The screen needs to be expanded to allow for true custom dice rolls.

Related: Eventually it would be nice to be able to type a roll formula directly in the chat, similar to Roll20 or Fantasy Grounds VTTs and have system roll the dice according to the formula, so that you don’t even have to open up a new screen to do a custom roll.