Only use buttons for submissions with reviews or comments

The rationale being that some people iteratively send in their
exercises, without awaiting comments. This makes it really hard to
judge if you're looking at submission 8 whether a whole discussion has
already been had, or whether no discussion has been had at all.


![screenshot](https://f.cloud.github.com/assets/624143/1832361/e3a8b068-73b1-11e3-8afa-3e4c95c11c5f.png)