Set Stripe customer ID into Account

[mblayman]

When the user signs up with school desk on a paid plan, a session completes and the Account is flipped to the ACTIVE state.

A customer cancelled and I discovered that the connection between the School Desk Account and Stripe Customer record is not made. The checkout.session.completed event that sets to ACTIVE should also set a (not yet existing) customer_id field on the Account.

This change will facilitate my ability to help customers.