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* NOTE `inveniomanage demosite create` is obsolete.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>
global: Invenio 1.9999.3.dev

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