Call different actions for standard and automated newsletters on Send step

- converted save and setStatus method of Newsletters endpoint
- updated pause/resume mixin for notification & welcome listings
- use 'newsletter_id' instead of 'id' in SendingQueue endpoint (less confusing)
- added NOT_FOUND constant to APIError
- fixed unit test for Newsletters endpoint save and added test for setStatus

lib/API/Error.php

@@ -8,6 +8,7 @@ final class Error {
   const BAD_REQUEST = 'bad_request';
   const UNAUTHORIZED = 'unauthorized';
   const FORBIDDEN = 'forbidden';
+  const NOT_FOUND = 'not_found';
 
   private function __construct() {