Add fc-item--fluid-width, initially used as the fallback for Thrashers

This adds a new item layout named `fc-item--fluid-width` to our repertoire. Initially this is only used in the new `fixed/thrasher` container and is the item before it's replaced (used as a fallback for no-js, if js fails or doesn't execute for whatever reason). However, I've made it generic item layout not attached to the new thrasher container, for future scalability.

cc @robertberry @sndrs @phamann 

Before:
![screen shot 2014-12-09 at 15 58 53](https://cloud.githubusercontent.com/assets/1607666/5360487/b1af011c-7fbc-11e4-9d2a-7da624f98b07.png)

After: 
![screen shot 2014-12-09 at 16 01 57](https://cloud.githubusercontent.com/assets/1607666/5360505/cbf89e3e-7fbc-11e4-92b2-99072f27056b.png)

wtf is `mf`?