Improve order notes endpoint

This commit improves the order notes endpoint with full read/write
capabilities:

* GET /orders/#{order_id}/notes - get all notes for an order
* GET /orders/#{order_id}/notes/#{id} - get single order note
* POST /orders/#{order_id}/notes - create an order note
* PUT /orders/#{order_id}/notes/#{id} - edit an order note
* DELETE /orders/#{order_id}/notes/#{id} - delete an order note
Merge pull request #5916 from maxrice/feature-rest-api-order-notes

Improve order notes endpoint