Show age for previous entry in user's time zone

Since we've merged https://github.com/codecation/trailmix/pull/124, I'm now seeing entries from exactly one week ago in each daily email. The entries are great, but the age message is incorrect. I'm seeing:

> Remember this? 8 days ago you wrote the following:

The reason this is happening is because in UTC land, my week old entry is actually 8 days old. But in PST land, where I live, it's 7 days old.

This PR makes it so that we use the user's time zone for the age message.
Does it need to be on user? Can't it just be a service class that accepts a user?