Partner Internal billing update

Partner: add billing type "Internal Billing"

Partner: add Addon Credit billing

Partner: Add Stripe Billing Portal for partner client

More details & tutorial here:

https://www.youtube.com/watch?v=6hA5w4MKKpo&list=PLYV-IhQ_GwTNojVoozTWF9sD1AQDBZL7Q