prepend the username in the participant alt-text in the topic list

Meta: [Alt-text for avatar images](http://meta.discourse.org/t/alt-text-for-avatar-images/6363)

This will prepend the username of the participant in the *alt-text* (`title` attribute) of the participants in the topic list view:

![image](https://f.cloud.github.com/assets/362783/464329/e32511ba-b5d7-11e2-8649-f718099dc632.png)