Separate twisted-specific configuration from WebConfigurationBase class
This is in preparation of dropping the Twisted web server backend.
We thus draw a clearer line between what's specific to twisted from
what's generic by moving WebConfigurationBase class into
cubicweb/web/webconfig.py and only keeping twisted-related in
AllInOneConfiguration (still living in cubicweb/etwist/twconfig.py).
cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)