migrate user/group management to use internal api

[utnapischtim]

The user/group management is now done with os.system calls of pipenv run invenio roles add [1] and the db queries [2]. This should be refactored to use internal api from invenio-accounts like [3] [4]. The refactoring is independent from #14

a further references is [5]

• [1] https://github.com/tu-graz-library/invenio-rdm-pure/blob/0888fa020f233c6b9abbf3e14b51c746fd12a4e6/invenio_rdm_pure/source/rdm/run/groups.py#L493
• [2] https://github.com/tu-graz-library/invenio-rdm-pure/blob/0888fa020f233c6b9abbf3e14b51c746fd12a4e6/invenio_rdm_pure/source/rdm/database.py#L23
• [3] https://github.com/inveniosoftware/invenio-accounts/blob/4b4182aac632e0dc1742644e3274cf85d456988f/invenio_accounts/models.py#L31
• [4] https://github.com/inveniosoftware/invenio-accounts/blob/4b4182aac632e0dc1742644e3274cf85d456988f/invenio_accounts/models.py#L49
• [5] https://github.com/inveniosoftware/flask-security-fork/blob/cb2b3bb080dde4ec7f207522ca7d901e095e5b84/flask_security/datastore.py#L148