Add reCAPTCHA check for creating new paste

[Albert221]

Our paste service has became a pretty friendly place for bots posting links to illegal movies. To prevent that, a anti-bot mechanism should be implemented. My choice is reCAPTCHA.

Implementing it should be fairly easy:

1. Add required front-end code to the home.twig view.
2. Add reCAPTCHA key sample(s) to the config.yml.example file.
3. Add check for the reCAPTCHA in paste creation validator.
   • You will need to refactor it a bit, so that it takes the mandatory keys using Dependency Injection (through the constructor).
   • Create this validator as a service in AppKernel.php.
   • Pass this newly created service to the PasteController

Here is some blog article that I quickly found on Google describing how to check the captcha: Implementing google reCAPTCHA on a form in symfony 3