Fix bug where input would scroll when typing

This should fix #366

After:
![chat-scroll](https://cloud.githubusercontent.com/assets/463175/5422284/83655b8c-8249-11e4-9ccc-f69412c70d83.gif)

Originally, the overflow-y was on .channels-pane, which contained both the chat box and the activities. Because of this, the chat box would steal focus from activities when you type.

This was explained perfectly in a stack overflow here:
https://stackoverflow.com/questions/11384116/on-webkit-browsers-typing-into-edit-box-causes-scrolling