Prevent publish-bar from floating up on mobile

This solves the issue where if the height of your screen is below a certain amount the publish bar scrolls up due to the editor input being too big.

### To reproduce

1. Open the editor in chrome
2. Hit the emulation button
3. Select a small device, like the iphone 4
4. Scroll up and down on the page (not the header)

![ghost-publish-bar](https://cloud.githubusercontent.com/assets/34726/5217784/1afdf01e-7613-11e4-867a-9b31b417425e.gif)