[not ready] Source UI adjustments

This is mostly ready. 

One hangup:
When loading or update a map, I have been running into stalls and there isn't good user feedback. This function runs: https://github.com/mapbox/mapbox-studio/blob/9c54cb1ce4b5d402cdc6ee48438acfe38662834b/app/source.js#L311-L380 and then I'll often get dead tiles or an unresponsive map. Things feel broken, but after a few seconds, everything snaps back into place. A loading indicator + better error reporting here would go a long way. 

Other todos: 

- [ ] Write tests
- [ ] Collect feedback ( @ian29 / @karenzshea / @ajashton ) anyone up for a quick test run? cmd+r for refresh, cmd+s for proper save.