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)

app/models/user.rb

@@ -245,7 +245,7 @@ class User < ApplicationRecord
   end
 
   def web_push_subscription(session)
-    session.web_push_subscription.nil? ? nil : session.web_push_subscription.as_payload
+    session.web_push_subscription.nil? ? nil : session.web_push_subscription
   end
 
   def invite_code=(code)