Configure sass

Looks like Dashbaord is already using sass/scss, so this is a no-brainer. This will let us split up the stylesheets to mirror the structure of any refactoring happening in templates & js code. Plus, variables! :-)

You'll need to run `npm install` after pulling these changes. Sadly, node-sass has a bunch of compiler warnings right now <https://github.com/andrew/node-sass/issues/137>, but they are harmless.

After that `grunt sass` is configured to run automatically with the build, as well as for watching/autoreload in the dev server.