assets: working combined assets

* Using STATIC_MAP to distinguish between modules and external
  resources.

* Adds STATIC_URL_PREFIX for prefixing all the static files in
  production mode. Useful if you have a dedicated host for static files.

* Fixing the documentation for sphinx.

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

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
docs: invenio.ext.assets added

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