fixes #3491 - API v2 rabl templates base, main, show for each controller

base.json.rabl - usually contains `:id, :name` and possibly another important attribute

main.json.rabl - extends 'base' and adds attributes, but no :child nodes
                         - index.json.rabl extends the 'main' template

show.json.rabl - extends 'main' and adds :child nodes
                          - currently, 'show' may not have anything different other than extending 'main', but this will probably change in PR 
                            fixes #3492 to add nested routes and relationships in responses.