Captcha/anti-bot: Altcha

Closes #38 (closed)

Experiments with Altcha

TODO:

• show nudge at the bottom right indicating when altcha is running
• add altcha to info panel
• altcha hmac server environment variable
• user reputation to determine alerts sent to staff (#171)
• remove old recaptcha implementation & environment variables
• [Mod UI] send captcha to user
• [Mod UI] send captcha to specific socket
• [Admin UI] altcha toggle (#170)
• [Admin UI] altcha parameters change (algorithm, maxnumber, salt) (#170)