admin: allow the creation and management of roles

[diegodelemos]

Currently, the admin user is just a hardcoded UUID.

$ rg ADMIN_USER_ID
reana-server/reana_server/reana_admin.py
29:from reana_server.config import ADMIN_EMAIL, ADMIN_USER_ID, REANA_HOSTNAME
69:@click.option("-i", "--id", "id_", default=ADMIN_USER_ID)
214:        admin = User.query.filter_by(id_=ADMIN_USER_ID).one_or_none()
279:        admin = User.query.filter_by(id_=ADMIN_USER_ID).one_or_none()

reana-server/reana_server/utils.py
38:from reana_server.config import ADMIN_USER_ID, REANA_GITLAB_URL
75:    admin = Session.query(User).filter_by(id_=ADMIN_USER_ID).one_or_none()
94:        admin = Session.query(User).filter_by(id_=ADMIN_USER_ID).one_or_none()
116:    admin = User.query.filter_by(id_=ADMIN_USER_ID).one_or_none()
136:    admin = User.query.filter_by(id_=ADMIN_USER_ID).one_or_none()

reana-server/reana_server/config.py
30:ADMIN_USER_ID = "00000000-0000-0000-0000-000000000000"

If we would like to extend REANA so to delegate quota management to subgroups (for example, an experiment spokesperson would be responsible for this) we would need real roles.

With these roles, we would be able to create an admin dashboard and show the quota information which reana-admin CLI shows on a web dashboard see reanahub/reana-ui#130.