Popupkit <= 2.2.0 - Missing Authorization to Authenticated (Subscriber+) Arbitrary Subscriber Data Deletion

The Popupkit plugin for WordPress is vulnerable to arbitrary subscriber data deletion due to missing authorization on the DELETE `/subscribers` REST API endpoint in all versions up to, and including, 2.2.0. This is due to the `permission_callback` only validating wp_rest nonce without checking user capabilities. This makes it possible for authenticated attackers, with Subscriber-level access and above, to delete arbitrary subscriber records.