captcha: add captcha widget to the signup window

[XiaochenCui]

This PR introduce Cloudflare Turnstile on signup window to prevent the abuse of free account.

How to use:

1. Create a "widget" at Cloudflare's Turnstile panel.
2. Add following config to volatile/config/config.json:

{
    "services": {
        ...
        "cloudflare-turnstile": {
            "enabled": true,
            "site_key": "<your site key>",
            "secret_key": "<your secret key"
        }
    },
    ...
}

Then the user will be validated by Cloudflare Turnstile during signup.

TODO

• Test the signup without "cloudflare-turnstile" config. (or "enabled" is false)
• Test "invisible" verification mode.
• Write the instruction for Cloudflare Turnstile's best practice.

[XiaochenCui]

@jelveh @KernelDeimos now it's ready for review & merge. I've tested it manually with different verification mode (managed/visible)

[XiaochenCui]

I'm not sure if we should use cloudflare-turnstile or cloudflare_turnstile as the config title