Change layout to maximize graph drawing area

[danielabar]

Move all menu interactions, controls etc to a single toolbar across the top of the page, and remainder should be for graph. There's not much now but Search, Undo/Redu, and a few interactions, but in the future, other options could be here such as Load a new project.

This may be a good opportunity to bring in a css framework for the layout, and introduce a sass step to the build. Also take out all the css imports in view or view models and centralize in a main.scss, with sass imports for each components. Because loading css via SystemJS and eventually getting it optimized may be a little sketchy.

Might also be useful to bring in a reset/normalize like this one

[danielabar]

Consider bringing in Bootstrap and Sass (or at least Sass). This PR has an example of modifying the Aurelia Navigation Skeleton to include a sass build step.