Refactor Stripe customers

The following discussion from !92 (merged) should be addressed:

• @kvainiute started a discussion:

  should this also be moved to customer service? especially since currently now we're creating a customer for every order, but ideally, in the future, we should have one-to-one relationship with registered user and customer

• Create a stripe customer based on session user

• Always verify if a customer already exists for the user before creation

• On stripe checkout, put the option to update the user with the new information