Extend the default `_footer.html` template

To add custom content to the default Datasette `_footer.html` template:

`templates/_footer.html`:
```html
{% extends "default:_footer.html" %} 
{% block footer_content %}
<div>
  <a href="/terms-conditions">Terms and Conditions</a>
  <a href="/privacy-policy">Privacy Policy</a>
</div>
{{ super() }} 
{% endblock %}
```

Start Datasette with: `datasette --template-dir templates/`