Bots easily bypass the simple `nonce` hack. This commit adds support
for the hcaptcha.com widget.
- New `Security` tab in the admin settings UI.
- Enable/disable CAPTCHA.
- Render CAPTCHA on the public subscription form.
Closes#1116.
This patch adds new array fields on `POST /tx`: `subscriber_emails[]`, `subscriber_ids[]`.
Either of these array fields can be sent with multiple subscribers.
The individual non-array fields `subscriber_id` and `subscriber_email` are deprecated.
Closes#994.
