[Usage-based] Untie paid Stripe subscriptions from calendar months

[jankeromnes]

Is your feature request related to a problem? Please describe

Until now, we force all Stripe subscriptions to go from calendar month to calendar month, instead of sign-up-date to sign-up-date + 1 month.

However, this is problematic for individual paid subscriptions, because of the 9€ flat fee.

Describe the behaviour you'd like

Thus, let's untie all Stripe paid subscriptions from calendar months, and instead let them go from sign-up-date to sign-up-date + 1 month.

Describe alternatives you've considered

• Pro-ration: Prorate final payment when individual users cancel their $€ 9.00 paid subscriptions. #13804 (unsupported in Stripe due to usage-based)
• Doing nothing (unfriendly and confusing for users)

Additional context

Steps:

• Don't specify a custom billing anchor date when creating new Stripe subscriptions
• Always specify both a billingCycleStart and nextBillingTime on all Cost Centers