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/initial_state_serializer.rb

@@ -2,7 +2,9 @@
 
 class InitialStateSerializer < ActiveModel::Serializer
   attributes :meta, :compose, :accounts,
-             :media_attachments, :settings, :push_subscription
+             :media_attachments, :settings
+
+  has_one :push_subscription, serializer: REST::WebPushSubscriptionSerializer
 
   def meta
     store = {