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

Given that the tokens we generate use `[a-zA-Z0-9\-\_]`, and the string is 8 chars long, that's a search space of `64 ** 8`.

Each time we create a user, we have a 1 in 281,474,976,710,656 chance of generating a duplicate token.

In other words, I think we can take that risk and just generate a token without worrying if it's a dupe. 

:)