assets: "less" flavor of bootstrap

* Installing less.
* Enabling filters on the assets `{% css "foo.css", "00-bundle", "less" %}`
* Adding one extra configuration key: `LESS_BIN` which must be pointed to lessc: e.g., `node_modules/grunt-contrib-less/node_modules/.bin/lessc`

Inspire are already using less but have to compile it everytime, this would make it all dynamic: https://github.com/inspirehep/inspire-next/tree/new-templates