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

I think the email spec says that emails are case sensitive, but I don't think they really are. If I send an email to `HuNtCa@gmail.com` and `huntca@gmail.com`, they go to the same place. I think the same applies for every other email provider. 

Devise also [ignores case of emails](https://github.com/plataformatec/devise/blob/8e5c098e3aa53d6a72faadd8b929abaf2bb6d496/lib/devise/models/authenticatable.rb#L65).

Sticking with one case on our reply emails seems consistent.