refactor: make stripe_operator.rs create CheckoutSessions function pre-invite

[skeptrunedev]

Current Flow

1. Stripe Customer is created when user sets their password
2. User signs in
3. User can request a CheckoutSession url for a given plan (Silver or Gold)
4. User goes to Stripe's hosted payment page and successfully pays or cancels
5. If cancels, goes back to /debate route
6. If succeeds, Stripe hits out webhook to tell us

New Flow

User does not have an account

• 1. User requests a CheckoutSession URL for a given plan (Silver or Gold)
• 2. User goes to Stripe's hosted payment page and successfully pays or cancels
• feature: webhook route that updates plan table when hit by stripe on successful payment #65
• 6. If succeeds, Stripe hits out webhook
• 7. We create an invite for the email on the payment
• 8. User receives email and sets their password

User already has an account

• 1. Logged in user requests a CheckoutSession URL for a given plan (Silver or Gold)
• 2. User goes to Stripe's hosted payment page and successfully pays or cancels
• 3. 5. If cancels, goes back to /debate route on app
• feature: webhook route that updates plan table when hit by stripe on successful payment #65
• 7. We mark the user's plan in the db
• 8. User is able to send more messages and use premium plan features