Implemented hCaptcha as an alternative to Google reCaptcha (Greenlight)

[Marvin]

Hello!

I have started this pull request to implement hCaptcha as an alternative to Google reCaptcha in Greenlight.

The operator of the server can setup hCaptcha instead of reCaptcha by setting two additional environment variables. The implementation of Google reCaptcha has of course remained. reCaptcha has priority if both services are configured at the same time.

hCaptcha is a captcha provider which is technologically similar to Google reCaptcha, but is much more privacy-oriented and actually rewards the website operator for embedding captchas. Site operators are rewarded with cryptocurrencies proportionately for each captcha solved. This is a great advantage, especially in light of Google's increasingly aggressive monetisation of reCaptcha.

Kind regards
Marvin