refactor the status widget

Some refactoring for CDE status:

- removed a bunch of hard-coded DOM manipulation
- the status widget moves into the bottom left area of the files view (no longer hovering over the editing area)
- changed to using css animations on an svg image, instead of using an animated gif (this works better with different backgrounds)
- used some flexbox layout stuff from polymer

@ussuri 
I'm experimenting w/ smaller components. I noticed in devtools that we pull a lot of css in from `common/spark_widget.css`. Don't know if this has a big performance hit or not -