Clear guestUsers, voiceCallStates and whiteboard-multi-user collections on meeting end

...ports/api/guest-users/server/modifiers/clearGuestUsers.js

.../imports/api/meetings/server/modifiers/meetingHasEnded.js

...oice-call-states/server/modifiers/clearVoiceCallStates.js

...d-multi-user/server/modifiers/clearWhiteboardMultiUser.js