## 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/views.py
is_multitenancy: bool = getattr(settings, "MULTI_TENANCY", False)
    if is_multitenancy and not request.user.is_staff:
        raise AuthenticationFailed(detail="You're not authorized.")
    from .models import Element, Event, SessionRecordingEvent
    redis_alive = is_redis_alive()
    postgres_alive = is_postgres_alive()
    metrics: List[Dict[str, Union[str, bool, int, float]]] = []
    metrics.append({"key": "posthog_version", "metric": "PostHog version", "value": VERSION})
    metrics.append(
        {
            "key": "analytics_database",
            "metric": "Analytics database in use",
            "value": "ClickHouse" if is_ee_enabled() else "Postgres",
        }
    )
    metrics.append({"key": "plugin_sever_alive", "metric": "Plugin server alive", "value": is_plugin_server_alive()})
    metrics.append(
        {
            "key": "plugin_sever_version",
            "metric": "Plugin server version",
            "value": get_plugin_server_version() or "unknown",
        }
    )
    metrics.append({"key": "db_alive", "metric": "Postgres database alive", "value": postgres_alive})
    if postgres_alive:
        postgres_version = connection.cursor().connection.server_version