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>
base: documentation fixes

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
grunt: jquery-migrate via bower

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