Add Sumly to a Django site

Set up Django for Sumly in this project.

Sumly (https://sumly.dev) shows revenue next to the visits and app users it came from. The full guide is https://sumly.dev/install/django; read it if anything below is unclear.

Steps:
1. In Sumly, open Visitors and copy your project's line.
2. Paste it into the `<head>` of your `base.html`, outside any `{% block %}`, so pages that extend it cannot replace it.
3. Deploy. Open the site and the visit shows on Visitors.

base.html: Plain HTML, outside the blocks.
```
<!DOCTYPE html>
<html lang="en">
<head>
    <title>{% block title %}My site{% endblock %}</title>
    <script defer src="https://sumly.dev/s.js" data-site="sl_your_key"></script>
</head>
<body>
    {% block content %}{% endblock %}
</body>
</html>
```

Checkout: For a Stripe Checkout your view creates: pass the visitor along.
```
session = stripe.checkout.Session.create(
    # …your line items, success_url and so on
    mode="subscription",
    subscription_data={"metadata": {"sumly_visitor_id": request.COOKIES.get("sumly_vid", "")}},
)
```

How to work:
- Look at how this project is built first, and fit the code to its framework and its way of doing things; the examples above are a starting point, not something to paste as is.
- Steps that happen in a dashboard (Sumly, or Django's own) are mine: tell me exactly what to click, then wait for me.
- Keys the code needs belong in environment variables, never in the code or the repository. Never ask me to paste a secret key into this chat; tell me which variable to set.
- Change only what this needs, show me the changes before you commit, and tell me how to check that it works.
- sl_your_key stands for the project's public key; it is on the Visitors page in Sumly. Ask me for it.