Support multiple classes per concept

From discussion on the dev list 30-April-2014, we could allow for multiple classes per concept in a backward-compatible way:

We keep concept.concept_class_id to identify the primary class for a concept, adding this table to store primary & any additional classes per concept:

concept_class_map
  concept_class_id
  concept_id

and methods like:

public Concept {
  ConceptClass[] getConceptClasses();
  void setConceptClasses(ConceptClass[]);
}

then refactoring the concept editing pages and ConceptService.getConcept(...) to support more than one class per concept would allow us to start recording more than one class per concept and leveraging them without requiring any changes to existing code. The transition to multiple-classes-per-concept could roll out to other parts of the application over time.