fixes https://github.com/TryGhost/Team/issues/1952
Adds a new MemberLinkClickEvent event that is fired when a member clicks a link. This code has been added to the `linkClickRepository` because that is the only place that has access to the member model (and the event requires the id and current last seen at value). The LastSeenAtUpdater listens for this event and updates the timestamp if required.
