Add db constraints to reply token

This is PR 2 of 4 for replacing `today@trailmix.life` with unique user emails:

1. #90 - Add reply token to User
2. **Add db constraints to reply token**
3. #92 - Start using reply token
4. #93 - Drop support for `today@trailmix.life`

After this is deployed, run the migration.

We can now move onto #92