base: fix static files serving from DocumentRoot

• Implements Nginx's "try_files" like functionality using custom wsgi middleware for Apache and WSGIScriptAlias for invenio.wsgi together with mod_rewrite.

• NOTE regenerate your apache configuration by running inveniomanage apache create-config.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Tested-by: Yoan Blanc <yoan.blanc@cern.ch>