Ask user which noble to choose if multiple are earned.

#### Test plan
Buy a bunch of cards until multiple nobles can be selected.

To make this easier:
```
// GameData.js
var nobles = [
  {cost: {black: 1, white: 1}, points: 3},
  {cost: {black: 1, red: 1}, points: 3},
};
```
Buy a red, a white, then a black card. Choose your noble. You should only get one.

nit: this function is more like selectableNobles