## 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.

#### *Thank you* for your bug report – we love squashing them!

posthog/views.py
response = {
        "django": True,
        "redis": is_redis_alive() or settings.TEST,
        "plugins": is_plugin_server_alive() or settings.TEST,
        "celery": is_celery_alive() or settings.TEST,
        "db": is_postgres_alive(),
        "initiated": User.objects.exists() if not settings.E2E_TESTING else False,  
        "cloud": settings.MULTI_TENANCY,
        "available_social_auth_providers": get_available_social_auth_providers(),
    }
    if request.user.is_authenticated:
        response = {
            **response,
            "ee_available": settings.EE_AVAILABLE,
            "ee_enabled": is_ee_enabled(),
            "db_backend": settings.PRIMARY_DB.value,
            "available_timezones": get_available_timezones_with_offsets(),
            "opt_out_capture": os.environ.get("OPT_OUT_CAPTURE", False),
            "posthog_version": VERSION,
            "email_service_available": is_email_available(with_absolute_urls=True),
            "is_debug": settings.DEBUG,
            "is_event_property_usage_enabled": settings.ASYNC_EVENT_PROPERTY_USAGE,
            "licensed_users_available": get_licensed_users_available(),
        }
    return JsonResponse(response)