refactor(payments-management): Refactor cancellation intervention

- This pull request
    Updates determineCancellationIntervention
        this method should determine which cancellation intervention flow (either churn or interstitial offer) should be presented to the customer when attempting to cancel
        remove the following as they will be required in determineCancelInterstitialOfferEligibility and determineCancelChurnContentEligibility
        fetch account customer by uid (and throw if the Stripe customer ids do not match
        retrieve the subscription
        retrieve subscription status
        check if subscription is active
        check if subscription is already canceling at period end
    Update determineCancelChurnContentEligibility and determineCancelInterstitialOfferEligibility as both should
        fetch the account customer and subscription (and throw if the Stripe customer ids do not match)
        retrieve the subscription
        retrieve subscription status
        check if subscription is active
        check if subscription is already canceling at period end
    Adds JSDoc to explain each responsibility

- Closes PAY-3493

Author: xlisachan