Making messagebox as responsive as message and user list.

Message box resizes to coincide with message and user list responsive layout.

Example of <480px width

Now

![Now](https://f.cloud.github.com/assets/77467/333962/b3ad7b90-9c67-11e2-861c-655f5f39d397.png)

Proposed

![Small screen message box](https://f.cloud.github.com/assets/77467/333937/6b8ccdc0-9c67-11e2-9f37-d2a86da0eac3.png)