Add UI to support initial health

[AlmostMatt]

Blocked by #61

[ajhyndman]

I had one further idea for how to approach this. We could let the UI show current HP, without trying to support editing. Then, instead, we could allow you, after teeing up an attack, let you "execute" that turn. Then your heroes hp and special cooldowns would be persisted, and you now have the option to preview what will happen on the next attack.

I kind of like this because it's intuitive, not fiddly, and is how state is changed in the real game. i.e. In the game you always start with full HP on each hero, and you only lose stats via an attack.

I think if you ever want to ask the question: "What happens if my Takumi is attacked while he is at 20hp?", you are really trying to answer the question: "What happens if my Takumi is attacked by Robin, and then attacked again by Cordelia?" I think this system could eliminate the manual transcription and state management.

[AlmostMatt]

My main complaint with that is that it feels a bit weird to 'apply' combats and 'reset' healths, and it also feels like extra work when a user just wants to test something like miracle, retribution, or vantage.

Also it doesn't work for the Ardent Sacrifice + Desperation combo.

[ajhyndman]

That's true, and that's a pretty neat combo, too.