Stubbed out Explorer Controllers

Just the basic routes, the output of rake routes is:

```text
    explorer_collection_documents GET    /explorer/:explorer_id/collections/:collection_id/documents(.:format)          explorer/documents#index
                                  POST   /explorer/:explorer_id/collections/:collection_id/documents(.:format)          explorer/documents#create
 new_explorer_collection_document GET    /explorer/:explorer_id/collections/:collection_id/documents/new(.:format)      explorer/documents#new
edit_explorer_collection_document GET    /explorer/:explorer_id/collections/:collection_id/documents/:id/edit(.:format) explorer/documents#edit
     explorer_collection_document GET    /explorer/:explorer_id/collections/:collection_id/documents/:id(.:format)      explorer/documents#show
                                  PUT    /explorer/:explorer_id/collections/:collection_id/documents/:id(.:format)      explorer/documents#update
                                  DELETE /explorer/:explorer_id/collections/:collection_id/documents/:id(.:format)      explorer/documents#destroy
             explorer_collections GET    /explorer/:explorer_id/collections(.:format)                                   explorer/collections#index
                                  POST   /explorer/:explorer_id/collections(.:format)                                   explorer/collections#create
          new_explorer_collection GET    /explorer/:explorer_id/collections/new(.:format)                               explorer/collections#new
         edit_explorer_collection GET    /explorer/:explorer_id/collections/:id/edit(.:format)                          explorer/collections#edit
              explorer_collection GET    /explorer/:explorer_id/collections/:id(.:format)                               explorer/collections#show
                                  PUT    /explorer/:explorer_id/collections/:id(.:format)                               explorer/collections#update
                                  DELETE /explorer/:explorer_id/collections/:id(.:format)                               explorer/collections#destroy
                   explorer_index GET    /explorer(.:format)                                                            explorer#index
                         explorer GET    /explorer/:id(.:format)                                                        explorer#show
```