Make operations on users resilient

The database and the Elasticsearch indices can be corrupted if an operation on users fails. The operation should be resilient to errors and do not persist inconsistent data.

Example :

1. POST api/users/create
2. user is created in database
3. commit
4. user is added to default group --> failure, for instance because the default group does not exist (bad configuration).
5. rollback, but it's too late, user has already been persisted and committed. User can't be created again and can't log in (no permissions)