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/workers/web/push_notification_worker.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class Web::PushNotificationWorker
+  include Sidekiq::Worker
+
+  sidekiq_options backtrace: true
+
+  def perform(subscription_id, notification_id)
+    subscription = ::Web::PushSubscription.find(subscription_id)
+    notification = Notification.find(notification_id)
+
+    subscription.push(notification) unless notification.activity.nil?
+  rescue Webpush::InvalidSubscription, Webpush::ExpiredSubscription
+    subscription.destroy!
+  rescue ActiveRecord::RecordNotFound
+    true
+  end
+end