Refactoring Stripe Customer Management

[qianl15]

• With this PR, instead of relying on Auth0 post login actions, we manage auth0 <-> Stripe customer ID mapping in our app's database through a reliable workflow. Specifically, we create a Stripe customer when a user first hits the subscribe/ endpoint, and record the user ID mapping in the accounts table.
• No longer record plan info in the table as it is redundant.
• Add more unit tests to show that we can test transactions individually.
• Update staging tests to also test subscribe/ and create-customer-portal endpoints.
• Clean up code so workflows are more readable.