New Interval + Logic Update

Previously there were some edge cases with order shipping dates when the shipping interval was based on a week and billing interval based on a month. A week doesn't exactly divide into a month so we have altered this to make sure shipments are always spread out by the exact shipping interval.

In addition to this we have added an new interval of "Every 4 Weeks" to allow for exact divisible cycles. E.g. Billing every 4 weeks and shipping every week.