refs https://github.com/TryGhost/Team/issues/2375
If a discount offer is associated with a tier that has a free trial enabled on full price / standard portal in membership settings, then the stripe checkout applied both the discount and free trial to the member, which is incorrect as we shouldn't be combining both.
- removes trial days from stripe checkout if a coupon is being applied, so only one of them is applied at a time
closes https://github.com/TryGhost/Team/issues/2222
Whilst we were checking for Stripe objects being active, we were not
checking for them existing in Stripe. This adds handling to all read
request to Stripe in the payment link flow, so that we can gracefully
handle deleted objects.
We've also included an automated test which fails without this fix.
We've also improved the query to find Stripe Prices which will result
in less request to the Stripe API to check if it is valid.
