Piccolo-admin

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JSON values are now displayed in a nicer format in the UI (courtesy sinisaos).

The popup banner displayed at the bottom of the page will now turn red when
showing an error (it was already green in the past). Courtesy sinisaos.

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``FormConfig.endpoint`` now works with async functions.

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Fixing a bug where setting ``FormConfig.description`` to ``None`` caused a
serialisation error.

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Added custom forms (courtesy sinisaos).

It's very easy to use - just provide a Pydantic model, and a function for
handling posted data. Piccolo Admin will then auto generate all of the UI
necessary.

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Using the ``swagger_ui`` endpoint from Piccolo API for the Swagger docs, so
it works with the CSRF middleware.

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Rewrote `admin_demo` command to expose configuration options on the command
line.

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