## Bug description

The SITE_ENV variable, which is quite useful for PostHog to function properly, is not mentioned in any of the deployment guides and only alluded to in the [Configuring Email](https://posthog.com/docs/configuring-posthog/email) page.

Apparently upgrading from one version of posthog to another broke some links in invite e-mails. I'm not sure, but I assume the e-mail sending code went into an async task, and as of such the URL couldn't be taken from `request` anymore, but reverted to `SITE_URL` that was unset. The e-mail config docs say to set this variable, but it might be easy to skip. Not sure.

## Expected behavior

I'd expect the site to give me a warning if I'm visiting `https://posthog.mysite.com/` while the `SITE_URL` variable is set to `http://localhost:8000/` or even `https://posthog.othersite.com/`. 

In an ideal world, I would see a warning like "You're browsing posthog from https://posthog.newsite.com/ yet your SITE_URL variable is set to https://posthog.oldurl.com/. Would you like to update it?" and a button to update it directly. Alternatively it would send me to a page on the docs that would describe the situation.

## How to reproduce

1. Install PostHog.
2. Either use a webhook or setup e-mail and invite an user (but forget to update SITE_URL)
3. Be confused as to why the link doesn't work.

## Environment

- [ ] PostHog Cloud
- [ ] self-hosted PostHog, version/commit: _please provide_

## Additional context

Was raised in the users' slack.

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posthog/api/test/test_preflight.py
def test_preflight_request_unauthenticated(self):
        """
        For security purposes, the information contained in an unauthenticated preflight request is minimal.
        """
        self.client.logout()
        with self.settings(MULTI_TENANCY=False):
            response = self.client.get("/_preflight/")
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        self.assertEqual(
            response.json(),
            {
                "django": True,
                "redis": True,
                "plugins": True,
                "celery": True,
                "db": True,
                "initiated": True,
                "cloud": False,
                "available_social_auth_providers": {"google-oauth2": False, "github": False, "gitlab": False},
            },
        )
    def test_preflight_request(self):