Break out touchList avatar to its own class, style to allow for icon avatar

This PR does two things:

1. Enforces the use of a `touchList-avatar` class for avatars inside touchLists. They are special.

2. More controversially: styles `touchList-avatar` so that it can be applied directly to a Red Label icon, e.g. `<i class="touchList-avatar icon icon-l icon-groups"></i>`, which will display like this:

    ![screen shot 2014-03-19 at 12 09 59 pm](https://f.cloud.github.com/assets/1885153/2462395/0234b1c0-af81-11e3-9eb2-06f26b7cfcea.png)

It's retina-friendly and keeps our visual voice consistent.