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/initializers/doorkeeper.rb

@@ -55,7 +55,7 @@ Doorkeeper.configure do
   # For more information go to
   # https://github.com/doorkeeper-gem/doorkeeper/wiki/Using-Scopes
   default_scopes  :read
-  optional_scopes :write, :follow
+  optional_scopes :write, :follow, :push
 
   # Change the way client credentials are retrieved from the request object.
   # By default it retrieves first from the `HTTP_AUTHORIZATION` header, then