Event Participant Update

Add a participant to an event or change an existing participant's state. If the requesting account is an admin or holds MANAGE_EVENTS permission, they can change the participation properties of any account. Otherwise, they can only change their own participation properties. For non-managing members (i.e. not an admin and not a host) this will follow a stricter state machine for the participation status. If the participation status is not set (no participation record is present) or set to "declined", the member may only set their status to "requested" if the event policy is set to "invite_only". Otherwise, they may set it to "attending". If the member is already set to one of these states, they may change it to "declined". A non-managing member cannot change their role and the default is "attendee", only managing members can change participant roles. If the event participation policy is set to "invite_only" then members can only set their status to "requested" or delete their participation. If the event participation policy is set to "closed", it's a no-op. Requests to this resource are idempotent given identical request bodies. It acts as a create-or-update action as participation is account-unique.