assets: bundles 2.0

• Adding a bundles jinja2 command to specify the required bundles.

• Supporting bundles definition using a module specific bundles.py file.

• Adding assets command to build the bundles from the command line.

• Adding a lazy registry to collect the bundles.

• Deprecating the old css,js,get_js_bundle,get_css_bundle jinja helpers.

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