get_role_assignements: use object's session, not app session
get_role_assignements: use object's session, not app session

fails in a celery task run from after_commit hook and celery configure in eager
mode (during dev and tests): app session is not usable during after_commit

abilian/abilian-core,abilian/abilian-core,abilian/abilian-core,abilian/abilian-core,abilian/abilian-core