New json api namespace

Adds a new public JSON api under the `/api` namespace.

It offers 10 endpoints for fetching pages, elements and contents as JSON objects:

```
                  api_contents GET    /api/contents(.:format)                                                             alchemy/api/contents#index {:format=>"json"}
                   api_content GET    /api/contents/:id(.:format)                                                         alchemy/api/contents#show {:format=>"json"}
          api_element_contents GET    /api/elements/:element_id/contents(.:format)                                        alchemy/api/contents#index {:format=>"json"}
           api_element_content GET    /api/elements/:element_id/contents/:name(.:format)                                  alchemy/api/contents#show {:format=>"json"}
                  api_elements GET    /api/elements(.:format)                                                             alchemy/api/elements#index {:format=>"json"}
                   api_element GET    /api/elements/:id(.:format)                                                         alchemy/api/elements#show {:format=>"json"}
             api_page_elements GET    /api/pages/:page_id/elements(.:format)                                              alchemy/api/elements#index {:format=>"json"}
       api_page_named_elements GET    /api/pages/:page_id/elements/:named(.:format)                                       alchemy/api/elements#index {:format=>"json"}
                     api_pages GET    /api/pages(.:format)                                                                alchemy/api/pages#index {:format=>"json"}
                      api_page GET    /api/pages/*urlname(.:format)                                                       alchemy/api/pages#show {:format=>"json"}
              api_preview_page GET    /api/admin/pages/:id(.:format)                                                      alchemy/api/pages#show {:format=>"json"}
```

This also deprecates the old json API, without `/api` namespace. The old api will be removed at next mayor release and will log deprecation warnings as long as we receive the next milestone.