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` (?)~~

![image](/uploads/d38ca89820d3c0066fb8aeb645fd77f0/image.png)

![image](/uploads/6b050749963691b023d076682abcf736/image.png)

Page when you fail CAPTCHA:

![image](/uploads/bc4846f0a5144985bc41dfa75eeab4c1/image.png)


See merge request !2216