Add stripe_id column to User

We'd like to allow users to close their account without having to send us an email at `team@trailmix.life`

Closing an account involves 2 steps:

1. Delete user information and entries from Trailmix
2. Delete customer record on Stripe

In order to delete the Stripe customer record, we [need the user's Stripe ID](https://stripe.com/docs/api#delete_customer).

This adds a `stripe_id` column to `User` so we can start persisting the user's Stripe ID. I'll follow up with other PRs to:

- Save the Stripe ID after signup for new users
- Back-fill Stripe IDs for customers that have already signed up
- Add `not null` constraint to `stripe_id`
- Allow users to close their account