Add initial Download Tab LESS file

Adds LESS file with imports, adds a new bundle in the jingo-minify config and calls `css()` helper in the template.

Also adds `normalize.css` and HTML5 Boilerplate's `main.css` to `media/css/libs` folder (other modules might want to use them). I wanted to go with `vendor` but that is in the `.gitignore`. I'm open to better ideas for the folder's name.