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>