## 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
from django.shortcuts import redirect
from django.views.decorators.cache import never_cache
from rest_framework.exceptions import AuthenticationFailed
from posthog.ee import is_ee_enabled
from posthog.email import is_email_available
from posthog.models import User
from posthog.utils import (
    get_redis_info,
    get_redis_queue_depth,
    get_table_approx_count,
    get_table_size,
    is_celery_alive,
    is_plugin_server_alive,
    is_postgres_alive,
    is_redis_alive,
)
from posthog.version import VERSION
from .utils import (
    get_available_social_auth_providers,
    get_available_timezones_with_offsets,
    get_celery_heartbeat,
    get_plugin_server_version,
)
def noop(*args, **kwargs) -> None:
    return None
try:
    from ee.models.license import get_licensed_users_available
except ImportError:
    get_licensed_users_available = noop
def login_required(view):