Comments on the Tarako review queue (bug 1053896)

https://bugzilla.mozilla.org/show_bug.cgi?id=1053896

@robhudson r?

![screenshot 2014-08-25 15 56 34](https://cloud.githubusercontent.com/assets/211578/4036482/52f58d64-2c9a-11e4-9851-7191b1731f21.png)

It allows null since when the developer creates the record they can't comment on their own review. `blank=True` is required for DRF to allow the reviewer to leave the comment blank.