TRUNK-3840: Refactoring code to display retired set member

Refactored concept.jsp; removed repeated code.
Add code to ConceptSet.java to factor in retired concepts while sorting
Added unit test for new behavior

Updated concept.jsp view and ConceptFormController

Factored in null weights

Changed ConceptSet compateTo method to use OpenmrsUtil's compateWithNullAsLowest
Follow up - TRUNK-3840