Add DB constraints to User#stripe_id

After https://github.com/codecation/trailmix/pull/112 has been deployed and the transition in https://github.com/codecation/trailmix/pull/113 has been performed, we can add some db constraints to `User#stripe_id`

The build will be red on this branch until we merge https://github.com/codecation/trailmix/pull/112.