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

Something I've been meaning to try with command objects such as this one:

```ruby
def self.generate(email)
  new(email).generate
end
```

Thoughts?