Fix notification attribution

Currently the notification emails misattribute nitpicks and likes. For example, I got an email today:
```
Hi ggilder,

You have 1 notification waiting for you at Exercism.io.

clojure: beer-song (http://exercism.io/submissions/199267359a0f727a2114e8d7)
nitpick from: ggilder about 10 hours ago
```
Needless to say, the nitpick was not actually from me. :wink: 

This patch includes a migration to store the creator of a notification, modifies the interface to `Notify.everyone` and `Notify.source` to require a creator, and modifies the `NotificationMessage` to properly render a notification's creator.

For backwards compatibility with previously created notifications that are missing a creator, I added a `from` helper method that outputs a blank string if the notification has no creator.