assets: working combined assets

* Fixing `static_url_path`, it now works as expected.

* 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.

* Adding config `instance_static_path` to identify where the generated
  static files are going. `static_folder` is used to generate the
  STATIC_MAP and has to represent the static files located in the
  modules.

* Relying on ASSETS_DEBUG for webassets debug mode.

* 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>