Clean up .avatar-list .username display

Re: #782, kind of.

There are always going to be usernames too long to display. This PR truncates them, and hides `.username` for `.avatar-list` that are not also `.avatar-list-lg`.

![image](https://cloud.githubusercontent.com/assets/1153134/5374350/a5b56e9a-8099-11e4-9658-79639be1f009.png)