Add reply token to User

This is PR 1 of 4 for replacing `today@trailmix.life` with unique user emails:

1. **Add reply token to User**
2. #91 - Add db constraints to reply token
3. #92 - Start using reply token
4. #93 - Drop support for `today@trailmix.life`

When this is deployed, all new Users will be created with a unique `reply_token`.

We must run the migration, then backfill existing users:

```ruby
> FillInReplyTokensTransition.new.perform
```

We can now move onto #91

The `before_create` ensures that new records get a reply token when created :smile: I have to set the column to `nil` after it's been created.