assets: working combined assets

• Fixing static_url_path feature.

• Fixing that all assets are created correctly using: url_for("static", filename="css/spam_and_eggs.css"). For specific modules, it's handy to give the module name as well, but it doesn't change anything, e.g., "annotations.static"

• Using STATIC_MAP to distinguish between modules and external resources.

• Using static_folder to point to the generated files. A hack is used to index the static files living in invenio.base.

• Relying on ASSETS_DEBUG for webassets debug mode.

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