The existing hCaptcha implementation as the only CAPTCHA option isn't ideal
as hCaptcha is a proprietary SaaS provider. This commit adds supports for
ALTCHA (altcha.org) a self-contained "proof-of-work" based CAPTCHA option.
Closes#2243.
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.
