Added recaptcha to prevent fake email pre registrations

[pragnankasarla]

I was testing out the website and found out that anyone can make pre-registrations without any authentication.

I was able to register 10 emails hello1@gmail.com, hello2@gmail.com,... and so on using a simple python script. In order to prevent this, I have added the Google Recaptcha before the register button in the pre-register dialog box.

Change the sitekey in pre-register-form.tsx file from 6LeIxAcTAAAAAJcZVRqyHh71UMIEGNQ_MXjiZKhI to your own public site key which you can generate for the specific domain using google recaptcha.

Here's a screenshot of how it looks like