Model chargedThroughDate as NextBillingPeriodStartDate

[mariogalic]

https://docs.google.com/document/d/1O-jIw0SBUcSjcLArZfWQY8ad2aX3H-b3O6ONrOQnFLQ

Holiday credit is applied to the next invoice on the first day of the next billing period.

This PR makes the above requirement more explicit by modelling the concept of next invoice like so

/**
 * Holiday credit is applied to the next invoice on the first day of the next billing period.
 *
 * 'Invoiced period' or `billing period that has already been invoiced` is defined as
 * [processedThroughDate, chargedThroughDate) meaning
 *   - from processedThroughDate inclusive
 *   - to chargedThroughDate exclusive
 *
 * Hence chargedThroughDate represents the first day of the next billing period. For quarterly
 * billing period this would be the first day of the next quarter, whilst for annual this would be
 * the first day of the next year.
 *
 * Note chargedThroughDate is an API concept. The UI and the actual invoice use the term 'Service Period'
 * where from and to dates are both inclusive.
 *
 * Note nextBillingPeriodStartDate represents a specific date yyyy-mm-dd unlike billingPeriod (quarterly)
 * or billingPeriodStartDay (1st of month).
 */
object NextBillingPeriodStartDate {

and making it part of the main for-comprehension in def processHolidayStop.

[coveralls]

Coverage increased (+0.04%) to 59.873% when pulling 2971a5d on factor-out-NextInvoiceStartDate into 3406350 on master.

[kelvin-chappell]

Might be better to call nextInvoiceStartDate nextBillingPeriodStartDate. And have some comments about what it means more explicitly and what its preconditions are. Eg. it only exists for a subscription when a bill run has happened.