Refactor /calendars into it's own controller

Spec it out too

config/routes.rb

@@ -198,6 +198,8 @@ Osem::Application.routes.draw do
     end
   end
 
+  resources :calendars, only: :index
+
   namespace :api, defaults: {format: 'json'} do
     namespace :v1 do
       resources :conferences, only: [ :index, :show ] do