Start using reply token

This is PR 3 of 4 for replacing `today@trailmix.life` with unique user emails:

1. #90 - Add reply token to User
2. #91 - Add db constraints to reply token
3. **Start using reply token**
4. #93 - Drop support for `today@trailmix.life`

In this phase, we start sending/processing emails using the reply token, but we still support `today@trailmix.life`. We need to support both for a while because folks will reply to older emails.

Once this has been deployed for a couple days, we can move on to #93
My preference is not to memoize unless we see that there's a performance problem in this class.

I think Rails is pretty smart about caching the `user` anyhow.