Add RecruitmentEventType to RecruitmentEvent

EPPs want to know what recruitment event is producing the most applicants/enrollees.  Currently there is only a RecruitmentTitle which is a free text field.  If we want to use this metric, would need to add a descriptor that would categorize the events into types so the metric would have some value.  

Need to identify the recruitment event types that would be included in the enums.  

Discussion in Model Review determined that this should be required and single instance, not collection