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
Rails doesn't cache `user` here because it's just a method. If we remove the memoization, we fetch the user from the database twice when we process an email:

```ruby
User Load (0.5ms)  SELECT  "users".* FROM "users"  WHERE "users"."reply_token" = 'user-1.ze9fkyepsio' LIMIT 1
User Load (0.3ms)  SELECT  "users".* FROM "users"  WHERE "users"."reply_token" = 'user-1.ze9fkyepsio' LIMIT 1
SQL (0.3ms)  INSERT INTO "entries" ("body", "created_at", "date", "updated_at", "user_id") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["body", "I am great"], ["created_at", "2014-10-04 16:57:18.411686"], ["date", "2014-10-04"], ["updated_at", "2014-10-04 16:57:18.411686"], ["user_id", 58]]
```

Since we use `user` in both `#process` and `#today`, memorization made more sense than passing the user around as an argument. We could move it up to the initializer instead if that reads better, but I'm not a huge fan of doing work in the initializer.