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/serializers/rest/web_push_subscription_serializer.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class REST::WebPushSubscriptionSerializer < ActiveModel::Serializer
+  attributes :id, :endpoint, :alerts, :server_key
+
+  def alerts
+    object.data&.dig('alerts') || {}
+  end
+
+  def server_key
+    Rails.configuration.x.vapid_public_key
+  end
+end