Datasette-auth-existing-cookies

- Fix for bug: `AttributeError: 'Datasette' object has no attribute '_auth_existing_cookies_cache'` [19](https://github.com/simonw/datasette-auth-existing-cookies/issues/19)

This alpha release breaks backwards compatibility, and completely redesigns how this plugin works. [13](https://github.com/simonw/datasette-auth-existing-cookies/issues/13)

- Plugin now uses the [actor_from_request()](https://docs.datasette.io/en/stable/plugin_hooks.html#plugin-hook-actor-from-request) plugin hook
- Implementation is greatly simplified: the forbidden mechanism and redirect to login via a signed `next=` parameter components have been removed
- Plugin configuration settings have been renamed, see [the README](https://github.com/simonw/datasette-auth-existing-cookies/blob/1.0a0/README.md#configuration)

* Respects `scope["auth"]` if it has already been populated by another authentication plugin - 12

If you are running multiple Datasette plugins this plugin will now attempt to go last and wrap all of the other plugins, ensuring they too will be protected by authentication.

The previous release always forwarded the `?host=` header on to the authentication API. This release no longer does that by default, but instead allows you to configure one or more headers that should be passed on in a similar way - issue 10.

To get back the same behavior as release 0.5, use this configuration:

json
{
"plugins": {
"datasette-auth-existing-cookies": {
"api_url": "http://www.example.com/user-from-cookies",
"auth_redirect_url": "http://www.example.com/login",
"original_cookies": ["sessionid"],
"headers_to_forward": ["host"]
}
}
}

Fix error triggered by combining this plugin with [datasette-upload-csvs](https://github.com/simonw/datasette-upload-csvs).