Create a customer for each Stripe Link payer

[devksingh4]

A customer should be identified by the email domain.

If the customer already exists, you should not create the customer. If the customer information has changed, then the user should be prompted to confirm to change the information.

We should store in our table the mapping from email to stripe customer ID. If it doesn't exist, then we create a customer ID in Stripe and store it.

We can store it in the same table as the payment history table, with the primary key CUSTOMER#<email domain> mapping to a stripe customer ID.

To make things easier to reconcile, the customer is specific to the org being paid and the email domain.