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/locales/doorkeeper.en.yml

@@ -115,5 +115,6 @@ en:
         title: OAuth authorization required
     scopes:
       follow: follow, block, unblock and unfollow accounts
+      push: receive push notifications for your account
       read: read your account's data
       write: post on your behalf