Global admins should not be added as group members

Currently, when we create a new instance of an orgunit, serie or event a group is created when none is specified and the current user is added to that group. In case the current user is a global admin, we probably shouldn't add him as the group api currently only deals with regular users when retrieving/updating members.
Not really, canAdminOrgUnit can be undefined if the user property is undefined. Undefined values are removed by express when serialised which leaves the orgunit witbout a `canManage` property