Improve configuration override

[Rakambda]

Description

Currently if we redefine only the amountCalculator in the predictions for a particular streamer, then the other values of the prediction section will be set back to default.

Example:
Global settings:

"predictions": {
          "amountCalculator": {
            "type": "percentage",
            "percentage": 0.05,
            "max": 500
          },
          "outcomePicker": {
            "type": "smart",
            "percentageGap": 0.05
          }
        }

Streamer settings:

  "predictions": {
    "outcomePicker": {
      "type": "smart",
      "percentageGap": 0.1
    }
  }

Currently the streamer would get outcomePicker as smart(0.1), but amountCalculator will be the default percentage(0.2, 50000).

I'd expect it to be outcomePicker as smart(0.1), and amountCalculator as percentage(0.0.5, 500) instead.