Align add-chat form elements with chat timeline

The size of the video preview and its alignment didn't flow well with the chats above it. I've adjusted the sizing + spacing to parity the chat items.

Before:

![image](https://f.cloud.github.com/assets/406639/1567007/ee0c2f04-5097-11e3-8d8e-a0e0eb2ce7ca.png)

After:

![image](https://f.cloud.github.com/assets/406639/1567017/2046b778-5098-11e3-8b32-43089b5e9c38.png)

At 320px wide:

![screen shot 2013-11-18 at 12 56 18 pm](https://f.cloud.github.com/assets/406639/1567019/2c0956d8-5098-11e3-9f68-7dc0b850eb54.png)