Cleaned add subscription prices for member details
Cleaned add subscription prices for member details

closes https://github.com/TryGhost/Team/issues/678
closes https://github.com/TryGhost/Team/issues/681

The prices in "Add subscription" modal should follow the same ordering as on the product detail screen, ie. currency, amount. Also, we only want to allow adding subscriptions for active prices, so the list is filtered on that. Since a Stripe customer is not allowed to have subscriptions in multiple currencies, this also filters the available currency prices based on any active subscription for a member.

TryGhost/Ghost-Admin,TryGhost/Ghost-Admin