Moved _tool.scss import from every tool's file into vizabi.scss

It seems unnecessary to import _tool.scss into every individual tool's .scss file, since it's only being extended there. This change removes ~2000 lines from the resulting vizabi.css file.

This PR is accompanied by a complimentary one I made to [generator-vizabi](https://github.com/Gapminder/generator-vizabi/pull/1).