installation: bower to manage the assets

* Defines .bowerrc for the static files.

* Adds base file to create the bower.json.

* Ignores the bower.json and vendors.

* Adds the Flask Debugtoolbar in extras_require.

* Adds setuptools-bower in extras_require.

* NOTE the demosite now requires Invenio==1.9999.1

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