Simplify BanditData model

[tomrf1]

The bandit data is fetched and cached regularly.
Originally this data was only used by the epsilon-greedy algorithm, which only requires the best variants (those with the highest mean). And so for efficiency the model just had a bestVariants array:

export interface BanditData {
    testName: string;
    bestVariants: BanditVariantData[]; // will contain more than 1 variant if there is a tie
}

We later added the "roulette" algorithm, which requires the means for all variants, so we added a variants field to the model:

export interface BanditData {
    testName: string;
    bestVariants: BanditVariantData[]; // will contain more than 1 variant if there is a tie
    variants: BanditVariantData[];
}

This PR simplifies the model to:

export interface BanditData {
    testName: string;
    sortedVariants: BanditVariantData[];  // sorted by mean, highest first
}

I'm doing this ahead of some work on simulation for selection methods.

[LAKSHMIRPILLAI]

👍🏼 Great