Add REST API for Web Push Notifications subscriptions (#7445)

- POST /api/v1/push/subscription
- PUT /api/v1/push/subscription
- DELETE /api/v1/push/subscription
- New OAuth scope: "push" (required for the above methods)

config/routes.rb

@@ -306,6 +306,10 @@ Rails.application.routes.draw do
       resources :lists, only: [:index, :create, :show, :update, :destroy] do
         resource :accounts, only: [:show, :create, :destroy], controller: 'lists/accounts'
       end
+
+      namespace :push do
+        resource :subscription, only: [:create, :update, :destroy]
+      end
     end
 
     namespace :web do