Confirm user by admin from the backend is broken since 13.3.0

[sgrossberndt]

The commit [TASK] replace die() with proper JsonResponse broke a part of the functionality to confirm a user by an admin from the backend:

The user is actually confirmed but the backend shows an HTTP 503 "Oops, an error occurred!" with content Syntax error for the admin who just confirmed.

This happens because invalid JSON is returned: the returned content does not only contain {"status":"okay"} but this is wrapped in the full html of the frontend page of the plugin.

This commit should be reverted.