Add better webhook support for Apple Subscriptions

[saraolson]

this will help us stay more accurate with sub status on our end

[saraolson]

one usecase to check when working on this:

• Has a gifted sub, buys a new Apple sub: should convert remaining time to banked months and start their new apple sub as an active recurring sub

[saraolson]

notes for another use case to check:

i downgraded from a 12mo recurring sub to a 1mo recurring sub. apple has me down as a 1mo recurring sub, but our UI still says 12mo.

im guessing because apple doesnt charge again until the original sub's timeframe has passed, but we should update it right away.

an hour later i opened the app again and it gave me an error 'SUBSCRIPTION_STILL_VALID' or somethin, then on close and reopen it cancelled my 12 mo sub. it looked like apple still had me down as a 1mo sub. when i tried to cancel, it then said i'd already cancelled.. so i tried to clear the sub from my account in settings and got 'subscription still valid'

account is magetest. it never switched to a 1mo sub on our end in all this.