mixer: new extension that uses Mixer library

* If the extension is enabled it will connect to database create and
  recreated commands so it can loads all the json files into the
  database.

* NOTE if this extension is enabled fixtures should not be
  used as probably there will be some database inconsistencies.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
fixtures: port to extension and signals usage

* NOTE `inveniomanage demosite create` is obsolete. All actions are
  executed during `{post,pre}_command` signal handlers.

Co-authored-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
access: removal of site specific configuration

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>