Reduce competing export actions

![share-source](https://cloud.githubusercontent.com/assets/83384/4040132/433dfb7a-2cdd-11e4-8ff5-e456d409d3e3.png)
![share-style](https://cloud.githubusercontent.com/assets/83384/4040133/433fd508-2cdd-11e4-8b26-11f83206ec4b.png)

Removes redundant export image button and puts emphasis on upload action over export. https://github.com/mapbox/mapbox-studio/issues/670

@samanpwbb @nickidlugash