assets: less flavor of bootstrap

• Installing less and grunt-contrib-less.

• Adding the filters parameter on the assets. {% css "foo.css", "00-bundle", "less" %}

• Adding some configuration keys. LESS_BIN which must be pointing to lessc. CLEANCSS_BIN ditto for CleanCSS LESS_RUN_IN_DEBUG (default True) to enable or disable the less bundling in DEBUG. ASSETS_DEBUG must be also set to True for it to work.

• Renamed base.css to base.less and it includes the less files from Bootstrap and Font-Awesome.

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