app: investigate factory pattern usage

[audrium]

It seems that factory.py and create_app() are only being used in reana-admin and scheduler (defined here). Needs a deeper investigation, but if that's the case there is no point in registering the blueprints there. Looks like invenio-base is taking care for starting up the application. To reproduce try to:
- comment/remove factory.py and app.py
- reana-dev docker-build -c . -b DEBUG=1
- reana-dev kind-load-docker-image -c .
- kill the old pod and inspect the new one - works as expected
- it does not work while redeploying the cluster because it can't start flask reana-admin create-admin-user

Stems from #417