Add support for Google reCAPTCHA in user registration to prevent spammers
To do:
- [x] Failing reCAPTCHA test causes all the fields to be lost
- ~~[ ] Improve styling of reCAPTCHA box~~ (not possible)
- ~~[ ] Put settings in `application_settings` (?)~~


Page when you fail CAPTCHA:

See merge request !2216