assets: less flavor of bootstrap

* Installing less and grunt-contrib-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

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>