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 
`common/spark_widget.css` itself is very small, but it imports Roboto woff2's, so that might be the reason. That is needed for the components to render the font properly, though. Or so I think... Did you make sure that SparkStatus uses Roboto with this change? If it does indeed, I'm curious where it gets it from, give that its CSS is encapsulated.