frab through 0.9.1 Privilege Escalation via conference_users_attributes in UsersController

frab through 0.9.1 contains a privilege escalation vulnerability that allows authenticated crew users to grant themselves orga roles by submitting unchecked nested conference_users_attributes. Attackers can abuse UsersController#update or #create, which skip filter_conference_users, to gain orga on any conference and fully manage its events, schedule, reviewers and crew.