Improve aggregation and minify files

Currently we do not minify any JS files. 😞 
One might suggest that we put the minification part in the themes build flow. But I suggest that we use [advagg](https://www.drupal.org/project/advagg) instead. It will minify all js files not only our themes. Also it will be more straight forward to setup with our current workflow using [tadaa](https://www.drupal.org/project/tadaa).
